3-Alpha-Hydroxysteroid Dehydrogenase Mutants and Process for the Preparation of Ursodeoxycholic Acid

ABSTRACT

The invention relates to novel 3-hydroxysteroid dehydrogenase mutants, to the sequences which code for these enzyme mutants, to processes for the preparation of the enzyme mutants and to their use in enzymatic conversions of cholic acid compounds, in particular in the preparation of ursodeoxycholic acid (UDCA); subject-matter of the invention is also novel processes for the synthesis of UDCA using enzyme mutants; and the preparation of UDCA using recombinant, multiply modified microorganisms.

The invention relates to novel 3α-hydroxysteroid dehydrogenase (3α-HSDH) mutants, in particular the 3α-HSDH of Comamonas testosteroni (formerly referred to as Pseudomonas testosteroni), to the sequences which code for these enzyme mutants, to processes for the preparation of the enzyme mutants and to their use in enzymatic conversions of cholic acid compounds, in particular in the preparation of ursodeoxycholic acid (UDCA); subject-matter of the invention is also novel processes for the synthesis of UDCA using enzyme mutants; and the preparation of UDCA using recombinant, multiply modified microorganisms.

BACKGROUND OF THE INVENTION

Bile acids are biomolecules which are required for the digestion and absorption of fats, fatty acids and hydrophobic vitamins. A bile acid which is found, in humans, in small amounts only is ursodeoxycholic acid (referred to as UDCA). It has recently gained great therapeutic importance in the dissolution of cholesterol-comprising gallstones. This compound is produced industrially in tonne quantity by chemical or enzymatic steps. An important precursor for the synthesis of UDCA is 12-ketoursodeoxycholic acid, which can be converted into UDCA by a Wolff-Kishner reduction. A route, described in the literature, for the synthesis of 12-ketoursodeoxycholic acid starts with cholic acid (3α,7α,12α-trihydroxy-5β-cholanic acid), which may be prepared by two oxidative steps which are catalyzed by 7α- and 12α-HSDHs, and one reductive step, catalyzed by a 7β-HSDH (Bovara R et al. (1996) A new enzymatic route to the synthesis of 12-ketoursodeoxycholic acid. Biotechnol. Lett. 18:305-308; Monti D et al. (2009) One-pot multienzymatic synthesis of 12-ketoursodeoxycholic acid: Subtle cofactor specificities rule the reaction equilibria of five biocatalysts working in a row. Adv. Synth. Catal. 351:1303-1311). A further route starts with 7-ketolithocholic acid, which may be converted into UDCA by stereoselectively reducing the 7-keto group; this step, too, is advantageously carried out with enzymatic catalysis, catalyzed by a 7β-HSDH (Higashi S et al. (1979) Conversion of 7-ketolithocholic acid to ursodeoxycholic acid by human intestinal anaerobic microorganisms: Interchangeability of chenodeoxycholic acid and ursodeoxycholic acid. Gastroenterologia Japonica 14:417-424; Liu L et al. (2011) Identification, cloning, heterologous expression, and characterization of a NADPH-dependent 7 beta-hydroxysteroid dehydrogenase from Collinsella aerofaciens. Appl. Microbiol. Biotechnol. 90:127-135.). A further advantageous synthetic route starts with dehydrocholic acid (DHCA), which may be converted into 12-ketoursodeoxycholic acid by two reductive steps; these two steps may be catalyzed by two stereoselective HSDHs (3α- and 7β-HSDHs) (Carrea G et al. (1992) Enzymatic synthesis of 12-ketoursodeoxycholic acid from dehydrocholic acid in a membrane reactor. Biotechnol. Lett. 14:1131-1135; Liu L et al. (2013) One-step synthesis of 12-ketoursodeoxycholic acid from dehydrocholic acid using a multienzymatic system. Appl. Microbiol. Biotechnol. 97:633-639).

The enzyme from C. aerofaciens has proved to be a very suitable 7β-HSDH. The gene sequence of this enzyme from C. aerofaciens is now known, so that firstly the enzyme can be made available recombinantly after cloning; secondly, it is possible to generate mutants of this enzyme by protein engineering methods and therefore optionally to find more advantageous enzyme variants, as may be the case.

The enzyme from C. testosteroni has proved to be a very suitable 3α-HSDH. The gene sequence of this enzyme is now known, so that firstly the enzyme can be made available recombinantly after cloning; secondly, it is possible to generate mutants of this enzyme by protein engineering methods and therefore optionally to find more advantageous enzyme variants, as may be the case.

The active substances ursodeoxycholic acid (UDCA) and its diastereomer chenodesoxycholic acid (CDCA) have, inter alia, been employed for many years as medicaments for the treatment of gallstone complaints. The two compounds differ merely by the configuration of the hydroxyl group on C atom 7 (UDCA: β-configuration, CDCA: α-configuration). To prepare UDCA, a variety of processes are described in the prior art, and these processes are carried out by purely chemical means or else as a combination of chemical and enzymatic process steps. The starting point is in each case cholic acid (CA), or CDCA, which is prepared starting from cholic acid.

Thus, the traditional chemical method for the preparation of UDCA can be shown diagrammatically as follows:

A severe disadvantage is, inter alia, the following: since the chemical oxidation is not selective, the carboxyl group and the 3α- and 7α-hydroxy group must be protected by esterification.

An alternative chemical/enzymatic process based on the use of the enzyme 12α-hydroxysteroid dehydrogenase (12α-HSDH) can be shown as follows and is described for example in PCT/EP2009/002190 of the present applicant.

Here, the 12α-HSDH oxidizes CA selectively to give 12-keto-CDCA. The two protective steps which are required by the traditional chemical method can be dispensed with here.

Furthermore, Monti, D., et al., (One-Pot Multienzymatic Synthesis of 12-Ketoursodeoxycholic Acid: Subtle Cofactor Specificities Rule the Reaction Equilibria of Five Biocatalysts Working in a Row. Advanced Synthesis & Catalysis, 2009) disclose an alternative enzymatic/chemical process, which can be shown diagrammatically as follows:

The CA is oxidized first by the 7α-HSDH enzyme from Bacteroides fragilis ATCC 25285 (Zhu, D., et al., Enzymatic enantioselective reduction of -ketoesters by a thermostable 7-hydroxysteroid dehydrogenase from Bacteroides fragilis. Tetrahedron, 2006. 62(18): p. 4535-4539) and 12α-HSDH to give 7,12-diketo-LCA. These two enzymes are in each case NADH-dependent. After the reduction by 7β-HSDH (NADPH-dependent) from Clostridium absonum ATCC 27555 (DSM 599) (MacDonald, I. A. and P. D. Roach, Bile induction of 7 alpha-and 7 beta-hydroxysteroid dehydrogenases in Clostridium absonum. Biochim Biophys Acta, 1981. 665(2): p. 262-9), 12-keto-UDCA results. A Wolff-Kishner reduction gives the end product. The disadvantage of this process is that a complete conversion is not possible due to the equilibrium situation of the catalyzed reaction, and that two different enzymes must be employed in the first step of the reaction, which makes the process more expensive. Lactate dehydrogenase (LDH; for regenerating NAD⁺) and glucose dehydrogenase (GlcDH or GDH, for regenerating NADPH) are employed for cofactor regeneration. The disadvantage of the cofactor regeneration used in that reaction is that the co-product which forms can only be removed with great difficulty from the reaction mixture, so that the reaction equilibrium cannot be influenced positively, which brings about incomplete conversion of the starting material.

A 7β-HSDH from the strain Collinsella aerofaciens ATCC 25986 (DSM 3979; previously Eubacterium aerofaciens) was described in 1982 by Hirano and Masuda (Hirano, S. and N. Masuda, Characterization of NADP-dependent 7 beta-hydroxysteroid dehydrogenases from Peptostreptococcus productus and Eubacterium aerofaciens. Appl Environ Microbiol, 1982. 43(5): p. 1057-63). Sequence information for that enzyme was not disclosed. The molecular weight as determined by gel filtration amounted to 45 000 Da (cf. Hirano, page 1059, left-hand column). Furthermore, the reduction of the 7-oxo group to the 7βp-hydroxy group could not be observed for said enzyme (cf. Hirano, page 1061, discussion, 1^(st) paragraph). A person skilled in the art can therefore see that the enzyme described by Hirano et al. is not suitable for catalyzing the reduction of dehydrocholic acid (referred to herein as DHCA) in the 7-position to give 3,12-diketo-7β-CA.

The applicant's earlier international patent application PCT/EP2010/068576 describes a novel 7β-HSDH from Collinsella aerofaciens ATCC 25986, which has, inter alia, a molecular weight (as determined by SDS gel electrophoresis) of approximately 28-32 kDa, a molecular weight (as determined by gel filtration, under non-denaturing conditions, such as, in particular without SDS): of approximately 53 to 60 kDa, and the ability of stereoselectively reducing the 7-carbonyl group of 7-keto-LCA to a 7β-hydroxy group.

Furthermore, PCT/EP2010/068576 provides a process for the preparation of UDCA which can be shown diagrammatically as follows:

Thus, CA is oxidized in a simple manner via the traditional chemical route. The DHCA is reduced by the enzyme pair 7β-HSDH and 3α-HSDH, individually one after the other or else in one pot, to give 12-keto-UDCA. In combination with Wolff-Kishner reduction, UDCA can thus be synthesized in only three steps, starting from CA. While the 7β-HSDH enzyme is dependent on the cofactor NADPH, the 3α-HSDH enzyme requires the cofactor NADH. The availability of enzyme pairs which are dependent on the same cofactor or with extended dependence (for example on the cofactors NADH and NADPH) would be advantageous because it could simplify cofactor regeneration.

WO 2012/080504 describes novel 7β-HSDH mutants from C. aerofaciens in the sequence region of the amino acid radicals 36 to 42 of the C. aerofaciens sequence, and biocatalytic processes for the preparation of UDCA, in particular also novel whole-cell processes, where 7β-HSDH and 3α-HSDH act together on the substrate.

WO 2011/147957 describes novel knock-out strains which are particularly suitable for the preparation of UDCA since it has been possible to switch off the undesired 7α-HSDH enzyme activity in targeted fashion.

Hwang et al. mention in PLOS ONE (2013), 8, 5, e63594 specific single mutants (P185A, G or W; T188A, S or W) and dual mutants (W173F/P185W and W173F/T188W) of the 3α-HSDH from C. testosteronii. These mutants, which possess N-His tags, are proposed for the NAD-dependent oxidation of the substrate androsterone. Other substrates are not discussed.

The problem of the present invention is the provision of further improved 3α-HSDHs. In particular, it was intended to provide enzyme mutants which can be employed even more advantageously for the enzymatic or microbial preparation of UDCA via the stereospecific reduction of DHCA 3-position, and which have in particular an improved activity for substrate and/or cofactor, and/or of a reduced substrate inhibition and/or altered cofactor utilization (increased, modified specificity or widened dependency).

SUMMARY OF THE INVENTION

Surprisingly, it was possible to solve the above problems by generating and characterizing improved mutants of 3α-HSDH from aerobic bacteria of the genus Comamonas, in particular of the strain Comamonas testosteroni, and by employing them in the conversion of cholic acid compounds, in particular the production of UDCA.

On the basis of structural and homology aspects, it has been attempted in accordance with the invention to define sequence regions which might be responsible for coenzyme binding or else for substrate recognition. This results in possibilities in modifying, in a targeted fashion, amino acids in these regions by means of mutagenesis so as to modify enzyme properties by such structural modifications. Thus, what is known as the “Rossmann fold”, which is responsible for coenzyme binding, in the region of the amino acids around approximately 8 to 68 in the case of the 3α-HSDH of Comamonas testosteroni. In accordance with the invention, it has now been attempted in particular to modify amino acids in this coenzyme binding region such that the enzyme accepts the NADPH instead of NADH. During the attempt of replacing the amino acid leucine in position 68 by alanine it has been found, surprisingly, that this 3α-HSDH mutant has a markedly higher activity. This has subsequently also been confirmed in as far as a plurality of further mutants, all of which had mutations in position 68, showed higher activities than the wild-type enzyme. Even enzyme mutants which had been purified to homogeneity and which had been compared with the correspondingly purified wild-type enzyme demonstrated markedly higher specific enzyme activities.

The improved activity can be identified clearly by the increase in the specific activity, in other words the activity value, based on the amount of protein, of the mutants 3α-HSDH [R34N], 3α-HSDH [R34C], 3α-HSDH [L68A], 3α-HSDH [L68C], 3α-HSDH [L68K], 3α-HSDH [L68S] and of the dual mutant 3α-HSDH [R34N/L68A]. An exchange of an amino acid is denoted by the spelling; for example, R34N means that the arginine (R) in position 34 has been replaced by asparagine (N). The 3α-HSDH which may be obtained from C. testosteroni, or this enzyme provided with a C-terminal hexahistidine (His) tag, is referred to as the wild-type enzyme.

Furthermore, there have been found, surprisingly, markedly improved specific activities in single mutants, such as, for example, 3α-HSDH [T188A], 3α-HSDH [T188S], and dual mutants such as, for example, 3α-HSDH [L68A/T188A] or 3α-HSDH [L68A/T188S], in respect of the substrates CDCA and/or NADH.

Furthermore, the above problem has been solved by providing a biocatalytic (microbial and/or enzymatic) process, comprising the enzymatic conversion of DHCA into 12-keto-UDCA via two reductive part-steps catalyzed by the 3α-HSDH mutants and 7β-HSDH described herein, which may occur simultaneously or staggered in any sequence, and cofactor regeneration by using dehydrogenases, which regenerate the consumed cofactor from both reductive part-steps.

DESCRIPTION OF THE FIGURES

FIG. 1a shows the amino acid sequence of the Collinsella aerofaciens 7β-HSDH (SEQ ID NO:2), and FIG. 1b shows the coding nucleic acid sequence (SEQ ID NO: 1) for the amino acid sequence of FIG. 1a ; FIG. 1c shows the amino acid sequence of the Comamonas testosteroni 3α-HSDH (SEQ ID NO: 4), and FIG. 1d the coding nucleic acid sequence (SEQ ID NO: 3) for the amino acid sequence of FIG. 1 c;

FIG. 2a shows the amino acid sequence of the Comamonas testosteroni 3α-HSDH; C-terminally extended by the His-Tag sequence LEHHHHHH; FIG. 2b shows the mutant 3α-HSDH [R34N] derived therefrom; FIG. 2c shows the mutant 3α-HSDH [L68A] derived therefrom; FIG. 2d shows the mutant 3α-HSDH [R34N/L68A] derived therefrom;

FIG. 3 shows the Michaelis-Menten kinetics of the purified 3α-HSDH with His tag for the substrate DHCA (top image) and for the cofactor NADH, respectively (lower image).

FIG. 4 shows the Michaelis-Menten kinetics of the alanine mutant [L68A] for the substrate DHCA (top image) and for the coenzyme NADH (lower image)

FIG. 5 shows the SDS gel of the 3α-HSDH [R34N/L68A] (image on the left) and the 3α-HSDH [L68A] (image on the right) and after expression in shake flask fermentation in LB. The soluble fraction applied is the cell-free crude extract. The 3α-HSDHs have a size of approx. 27.4 kDa. Approximately 10 μg of protein were applied. Key: 1=purified 3α-HSDH [R34N/L68A]; 2=crude extract 3α-HSDH [R34N/L68A]; 3=purified 3α-HSDH [L68A]; 4=crude extract 3α-HSDH [L68A]

FIG. 6 shows the Michaelis-Menten kinetics of the dual mutant [R34N/L68A] for the substrate DHCA (top image) and for the coenzyme NADH (lower image)

FIG. 7 shows the comparison of the kinetic constants of wild-type enzyme, of the mutant L68A and of the dual mutant R34N/L68A. (Substrate DHCA: top image and coenzyme NADH: lower image)

FIG. 8 shows the Michaelis-Menten kinetics of the wild-type 3α-HSDH for the substrate DHCA, recorded in each case with crude extract. The curve with points shows the dependency of the reaction rate on the substrate concentration for the enzyme without His tag, the curve with triangles for the enzyme with C-terminal His tag.

FIG. 9 shows the SDS gel of the 3α-HSDH with 3×His and 9×His tag (image on the left) and the standard 6×His tag (image on the right). The expression was carried out in shake flask fermentation in LB medium. The soluble fraction applied is the cell-free crude extract and the purified protein. The 3α-HSDH 3×His has a size of approx. 27.1 kDa, the 6×His a size of approx. 27.5 kDa and the 9×His a size of approx. 27.9 kDa. Approximately 10 μg of protein were applied.

SPECIFIC EMBODIMENTS OF THE INVENTION

In particular, the invention relates to the following specific embodiments: 1. 3α-hydroxysteroid dehydrogenase (3α-HSDH), which catalyzes at least the stereospecific enzymatic reduction of a 3-ketosteroid to the corresponding 3-hydroxysteroid (such as, for example, of 3,12-diketo-7β-CA or DHCA to 12-keto-UDCA or 7,12-diketo-3α-CA (7,12-diketo-LCA), in particular under stoichiometric consumption of NADH and/or NADPH, in particular NADH), wherein the enzyme comprises a mutation at position 34 and/or 68 of SEQ ID NO:4 or at the corresponding sequence positions of an amino acid sequence derived therefrom with at least 80% sequence identity, such as, for example, at least such as, for example, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 or 99.5% sequence identity to SEQ ID NO:4; and/or C-terminally includes a chain extension by up to 25, in particular by up to 15, 14, 13, 12, 11 or 10 identical or different (proteinogenic) amino acid radicals, in particular by a peptide chain comprising 1 to 10, such as, for example, 3 to 9, basic amino acid radicals, such as lysine, arginine or in particular histidine (such as 3×His, 6×His or 9×His); it being possible for the histidine residues to be bonded directly to the C-terminus or via a linker group comprising any 1 to 10, in particular 1 to 3, amino acid radicals. Examples which may be mentioned are: LEHHH, LEHHHHHH and LEHHHHHHHHH. Subject matter are furthermore those 3α-HSDHs which additionally to one or more of the above mutations, or as an alternative thereto, include an N-terminal chain extension by up to 25 identical or different amino acid radicals. These are extended by up to 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11 or 10 identical or different (proteinogenic) amino acid radicals, in particular by a peptide chain comprising 1 to 10, such as, for example, 3 to 9, basic amino acid radicals, such as lysine, arginine or in particular histidine (such as 3×His, 6×His or 9×His); it being possible for the histidine residues to be bonded directly to the N terminus or via a linker group comprising any 1 to 10 amino acid radicals. An example which may be mentioned is: MGSSHHHHHHSSGLVPRGSH-.

For example, preferred 3α-HSDHs include a mutation at position 34 and/or 68 and are optionally additionally N-terminally or C-terminally extended; or they are only N-terminally or C-terminally extended; in each case as defined herein in greater detail.

Furthermore, the invention comprises native, unmutated enzyme as per SEQ ID NO:4, which is only N- or C-terminally modified as described hereinabove.

2. 3α-HSDH, which catalyzes at least the stereospecific enzymatic reduction of a 3-ketosteroid to the corresponding 3-hydroxysteroid, and which has an amino acid sequence which is modified by amino acid mutation in SEQ ID NO:4, the amino acid sequence mutation being selected among single or multiple mutations comprising:

-   -   a. R34X₁ and/or     -   b. L68X₂         where X₁ represents an amino acid radical which is other than         arginine (R), in particular a proteinogenic amino acid radical,         in particular an amino acid which increases any specific         activity and/or which reduces substrate inhibition and/or which         modifies cofactor utilization or cofactor dependency, in         particular a natural amino acid;         is; and         X₂ represents a proteinogenic amino acid radical which is other         than leucine (L), in particular an amino acid which increases         any specific activity and/or which reduces substrate inhibition         and/or which modifies cofactor utilization or cofactor         dependency, in particular a natural amino acid;     -   c) addition mutants which are extended C-terminally by a chain         comprising 1 to 10 histidine (H) residues; it being possible for         example for the histidine residues to be bonded directly to the         C terminus or via a linker group comprising 1 to 10, in         particular 1 to 3, amino acid radicals.     -   d) addition mutants which are extended N-terminally by 1 to 10         histidine (H) residues; it being possible for example for the         histidine residues to be bonded directly to the C terminus or         via a linker group comprising 1 to 10 amino acid radicals.         the mutated, i.e. modified, amino acid sequence having a         sequence identity to SEQ ID NO:4 of 80% to less than 100%, such         as, for example, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 or         99.5% sequence identity.     -   3. 3α-HSDH as per one of the preceding claims, selected among     -   a) the single mutants     -   R34X₁ and     -   L68X₂ and the     -   b) the dual mutants     -   R34X₁/L68X₂,     -   wherein     -   X₁ represents N, C, S or L;     -   X₂ represents A, G, C, K, S or V,     -   wherein particularly preferred combinations of X1, X2; comprise         X₁=N and/or X₂=A     -   c) addition mutants which are extended C-terminally by a peptide         chain comprising 1 to 10 histidine residues; it being possible         for example for the histidine residues to be bonded directly to         the C terminus or via a linker group comprising 1 to 10, in         particular 1 to 3, amino acid radicals;     -   d) addition mutants which are extended N-terminally by a peptide         chain comprising 1 to 10 histidine residues; it being possible         for example for the histidine residues to be bonded directly to         the N terminus or via a linker group comprising 1 to 10 amino         acid radicals and     -   e) mutants as per a) and b), in each case additionally         comprising the addition mutation as per c) and/or d), in         particular as per c) or d).         Nonlimiting examples of suitable mutants are         single mutants comprising: [R34N], [R34C], [R34S], [R34L],         [L68A], [I68C], [L68K], [L68S], [L68V] and [L68G]         dual mutants comprising: [R34N/L68A]         in each case both without His tag and with C-terminal and/or         N-terminal His tag.         4. 3α-HSDH as per one of the previous claims, which, in         comparison with the native, unmutated 3α-HSDH with SEQ ID NO:4         shows the following property profile:     -   a. an increased specific activity (Vmax [U/mg]) for         dehydrocholic acid (DHCA) in the enzymatic reduction of DHCA         with NADH as cofactor; wherein, for example, the specific         activity (U/mg) in the presence of the cofactor NAD(P)H in         comparison with the unmutated enzyme is increased by at least 1,         5, 10, 50 or 100%, but in particular at least by 1-fold, in         particular by 2- to 100-fold or 3- to 20-fold or 5- to 10-fold,     -   b. a reduced substrate inhibition by DHCA, such as, for example,         with Ki values in the range of from >10 mM, such as, for         example, at 11 to 200 mM, 12 to 150 mM, 15 to 100 mM,     -   c. it being possible for these properties a) and b) to be         present individually or in any combination, i.e. only a), only         b), only c), a) and b), a) and c) or b) and c).         5. 3α-hydroxysteroid dehydrogenase (3α-HSDH) which catalyzes at         least the stereospecific enzymatic reduction of a 3-ketosteroid         to the corresponding 3-hydroxysteroid (such as, for example, of         3,12-diketo-7β-CA or DHCA to 12-keto-UDCA or 7,12-diketo-3α-CA         (7,12-diketo-LCA), in particular with stoichiometric consumption         of NADH and/or NADPH, especially with stoichiometric consumption         of NADH, wherein the enzyme includes a mutation at position 188         and/or position 185 and optionally position 68 of SEQ ID NO:4 or         at the corresponding sequence positions of an amino acid         sequence with at least 80%, such as, for example, at least such         as, for example, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 or         99.5% sequence identity to SEQ ID NO:4; and/or which includes a         C-terminal chain extension by up to 15, 14, 13, 12, 11 or 10         identical or different (proteinogenic) amino acid radicals, in         particular by a peptide chain comprising 1 to 10, such as, for         example, 3 to 9, basic amino acid radicals, such as lysine,         arginine or in particular histidine (such as 3×His, 6×His or         9×His); it being possible for the histidine residues to be         bonded directly to the C terminus or via a linker group         comprising any 1 to 3 amino acid radicals. Examples which may be         mentioned are: LEHHH, LEHHHHHH and LEHHHHHHHHH. Subject matter         are furthermore those 3α-HSDHs which additionally to one or more         of the above mutations, or as an alternative thereto, include an         N-terminal chain extension by up to 25 identical or different         amino acid radicals. These are extended by up to 25, 24, 23, 22,         21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11 or 10 identical or         different (proteinogenic) amino acid radicals, in particular by         a peptide chain comprising 1 to 10, such as, for example, 3 to         9, basic amino acid radicals, such as lysine, arginine or in         particular histidine (such as 3×His, 6×His or 9×His); it being         possible for the histidine residues to be bonded directly to the         N terminus or via a linker group comprising any 1 to 10 amino         acid radicals. An example which may be mentioned is:         MGSSHHHHHHSSGLVPRGSH-.

For example, preferred 3α-HSDHs each include a mutation at position 188 and/or 185 and optionally at position 68 and are optionally additionally N-terminally or C-terminally extended; or they are only N-terminally or C-terminally extended; in each case as defined herein in greater detail.

6. 3α-HSDH which catalyzes at least the stereospecific enzymatic reduction of a 3-ketosteroid to the corresponding 3-hydroxysteroid and which includes an amino acid sequence which is modified by amino acid mutation in SEQ ID NO:4, the amino acid sequence mutation being selected among single or multiple mutations comprising:

-   -   a) P185X₁ and/or     -   b) T188X₃     -   and optionally additionally     -   c) L68X₂     -   wherein X₁ represents a (proteinogenic) amino acid radical which         is other than proline (P);     -   X₂ represents a (proteinogenic) amino acid radical which is         other than leucine (L);     -   X₃ represents a (proteinogenic) amino acid radical which is         other than threonine (T);     -   d) optionally additionally with C-terminal extension comprising         1 to 10 histidine (H) residues; and/or optionally additionally         with N-terminal extension comprising 1 to 10 histidine (H)         residues.         wherein the mutated, i.e. modified, amino acid sequence has a         sequence identity to SEQ ID NO:4 of 80% to less than 100%, such         as, for example, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 or         99.5% sequence identity.         7. 3α-HSDH as per one of embodiments 5 and 6, selected among     -   a) the single mutants     -   P185X₁     -   T188X₃     -   b) the dual mutants     -   P185X₁/T188X₃;     -   L68X₂/T188X₃;     -   L68X₂/T185X₁;     -   wherein     -   X₁ represents A, T, S, G or V, in particular A;     -   X₂ represents A, G, C, K, S or V, in particular A; and     -   X₃ represents A, G, W, S or V, in particular A or S;     -   c) optionally additionally with C-terminal extension comprising         1 to 10 histidine residues, it being possible for example for         the histidine residues to be bonded directly to the C terminus         or via a linker group comprising 1 to 10 amino acid radicals.     -   and; and/or     -   d) optionally additionally with N-terminal extension comprising         1 to 10 histidine residues, it being possible for example for         the histidine residues to be bonded directly to the N terminus         or via a linker group comprising 1 to 10 amino acid radicals.     -   and     -   e) mutations as per a) and b), in each case additionally         comprising the addition mutation as per c) and/or d), in         particular as per c) or d).

Those which must be mentioned in particular are the dual mutants P185X₁/T188X₃; L68X₂/T188X₃; L68X₂/T185X₁; without or with N- or C-terminal extension as defined hereinabove.

Nonlimiting examples of suitable mutants are:

single mutants comprising: [T188A], [T188S], [T188G], [T188V], [T188W], [P185A], [P185T], [P185S], [P185G], [P185V], [P185A], dual mutants comprising: [L68A/T188A] and [L68A/T188S], in each case both without His tag and with C-terminal and/or N-terminal His tag. 8. 3α-HSDH as per one of the embodiments 5 to 7 which shows the following property profile in comparison with the native, unmutated 3α-HSDH with SEQ ID NO:4:

-   -   a. an increased specific activity (v_(max) [U/mg]) for         dehydrocholic acid (DHCA) in the enzymatic reduction of DHCA         with NADH as cofactor; wherein, for example, the specific         activity (U/mg) in the presence of the cofactor NADH in         comparison with the unmutated enzyme is increased by at least 1,         5, 10, 50 or 100%, but in particular at least by 1-fold, in         particular by 2- to 100-fold or 3- to 20-fold or 5- to 10-fold,     -   b. a reduced substrate inhibition by DHCA, such as, for example,         with Ki values in the range of from >10 mM, such as, for         example, at 11 to 200 mM, 12 to 150 mM, 15 to 100 mM,     -   c. an increased specific activity (v_(max) [U/mg]) for NADH in         the enzymatic reduction of DHCA with NADH as cofactor, wherein,         for example, the specific activity (U/mg) in the presence of the         DHCA in comparison with the unmutated enzyme is increased by at         least 1, 5, 10, 50 or 100%, but in particular at least by         1-fold, in particular by 2- to 100-fold or 3- to 20-fold or 5-         to 10-fold,     -   it being possible for these properties a) to c) to be present         individually or in any combination, i.e. only a), only b), only         c), a) and b), a) and c) or b) and c).         9. Nucleotide sequence which codes for a 3α-HSDH as per one of         the preceding embodiments.         Examples which may be mentioned are nucleic acid sequences         selected among nucleic acid sequences         a) simultaneously coding for a GDH and a 3α-HSDH mutant as per         one of the embodiments 1 to 8 and optionally a 7β-HSDH;         b) coding for a fusion protein comprising a GDH and a 3α-HSDH         mutant as per one of the embodiments 1 to 8 and optionally a         7β-HSDH;         where the coding sequences independently of one another may be         present singularly or multiply in the construct, such as, for         example, in 2, 3, 4, 5, or 6 to 10 copies. Thus, any existing         differences in the activity of the individual expression         products may be compensated for by choosing the suitable copy         number.         10. Expression cassette, comprising at least one nucleotide         sequence as per embodiment 9 under the control of at least one         regulatory sequence and optionally coding sequences for at least         one (such as, for example, 1, 2 or 3) further enzyme, selected         among hydroxysteroid dehydrogenases, in particular 7β-HSDH, and         dehydrogenases which are suitable for cofactor regeneration,         such as, for example, FDH, GDH, ADH, G-6-PDH, PDH. In         particular, the enzymes which are present in an expression         cassette may utilize different, but preferably identical, pairs         of cofactors, such as, for example, the pair of cofactors         NAD⁺/NADH or NADP⁺/NADPH.         11. Expression vector, comprising at least one expression         cassette as per embodiment 10.         12. Recombinant microorganism which bears at least one         nucleotide sequence as per embodiment 9 or at least one         expression cassette 10 or at least one expression vector as per         embodiment 11.         13. Recombinant microorganism as per embodiment 12 which         additionally optionally bears the coding sequence for at least         one further enzyme, selected among hydroxysteroid dehydrogenases         (HSDH) and dehydrogenases which are suitable for cofactor         regeneration.         14. Recombinant microorganism as per embodiment 13, the further         HSHD being selected among 7β-HSDHs; and         the dehydrogenase is selected among NADPH-regenerating enzymes,         such as NADPH dehydrogenases, alcohol dehydrogenases (ADH), and         NADPH-regenerating formate dehydrogenases (FDH), and also         glucose dehydrogenase (GDH), glucose-6-phosphate dehydrogenase         (G-6-PDH) or phosphite dehydrogenases (PtDH), or         NADH-regenerating enzymes, such as NADH dehydrogenases,         NADH-regenerating formate dehydrogenases (FDH),         NADH-regenerating alcohol dehydrogenases (ADH),         NADH-regenerating glucose-6-phosphate dehydrogenases (G6PDH),         NADH-regenerating phosphite dehydrogenases (PtDH) and         NADH-regenerating glucose dehydrogenases (GDH).

An example which may be mentioned is a recombinant microorganism which is capable of simultaneously expressing an inventive 3α-HSDH mutant, a herein described GDH and optionally a herein described 7β-HSDH.

Recombinant microorganism which bears the coding sequences for 3α-HSDH mutant, GDH or mutants thereof and 7β-HSDH and one or more (different) expression constructs. Subject-matter of the invention is therefore recombinant microorganisms which are modified (such as, for example, transformed) with a single-plasmid system which bear the coding sequences for 3α-HSDH mutant, GDH or mutants thereof and 7β-HSDH or mutants thereof in one or more copies, such as, for example, 2, 3, 4, 5 or 6 to 10 copies. Subject-matter of the invention is therefore also recombinant microorganisms which are modified (such as, for example, transformed) with a single-plasmid system which bear the coding sequences for 7β-HSDH or mutants thereof, GDH or mutants thereof and 3α-HSDH or mutants thereof in one or more copies such as, for example, in 2, 3, 4, 5 or 6 to 10 copies. The enzymes (7β-HSDH, GDH and 3α-HSDH or their mutants) may, however, also be present in one or more copies on 2 or 3 separate plasmids which are compatible with each other. Suitable basic vectors for the preparation of single-plasmid systems and multicopy plasmids are known to the person skilled in the art. Examples which may be mentioned for a single-plasmid system are, for example, pET21a, and for multicopy plasmids for example the Duet vectors, which are available from Novagen, such as pACYCDuet-1, pETDuet-1, pCDFDuet-1, pRSFDuet-1 and pCOLADuet-1. Such vectors, their compatibility with other vectors and microbial host strains can be found for example in the “User Protocol” TB340 Rev. E0305 from Novagen.

The optimal combination of enzymes for generating plasmid systems may be done by the person skilled in the art without undue burden, taking into consideration the teaching of the present invention. Thus, for example, the person skilled in the art may select, for example as a function of the cofactor specificity of the HSDH enzyme used in each case, the enzyme which is best suited to cofactor regeneration, selected among the abovementioned dehydrogenases, in particular GDH and the respective mutants thereof.

Furthermore, it is possible to distribute the enzymes chosen for the conversion to two or more plasmids, and to prepare, using the plasmids thus prepared, two or more different recombinant microorganisms which are then employed together for the inventive biocatalytic conversion. The respective enzyme combination used for preparing the plasmid may, in this context, in particular also take place with the requirement of a comparable cofactor utilization. Thus, for example, a first microorganism may be modified with a plasmid which bears the coding sequence for a 7β-HSDH or mutant thereof and a GDH. A second microorganism, in contrast, may be modified with a plasmid which bears the coding sequence for a 3α-HSDH or mutant thereof and the coding sequence for a GDH. Both pairs of enzymes may be chosen such that they are capable of regenerating identical pairs of cofactors. Both microorganisms can then be employed simultaneously for the inventive biocatalytic conversion.

The use of two separate biocatalysts (recombinant microorganisms) may have two essential advantages over the use of only one biocatalyst in which all enzymes for the synthesis are expressed:

a) both biocatalysts may be genetically modified and optimized separately from one another. In particular, it is possible to use various cofactor regeneration enzymes, which are optimized either for the regeneration of NADH or a NADPH. b) It is possible to employ the biocatalysts in differing proportions for the biocatalysis. This allows engaging in the individual reaction rates of the multienzyme process during biocatalysis, even after all the biocatalysts have already been prepared. 15. Recombinant microorganism as per one of embodiments 12 to 14, which is a 7α-HSDH knock-out strain, wherein it the strain, as is described, for example, in WO 2011/147957. 16. Process for the enzymatic or microbial synthesis of 3α-hydroxysteroids, wherein the corresponding 3-ketosteroid is reduced in the presence of a 3α-HSDH as per the definition in one of embodiments 1 to 8 or in the presence of a recombinant microorganism which expresses this 3α-HSDH as per one of embodiments 12 to 15, and optionally at least one formed reduction product is isolated from the reaction mixture. 17. Process as per embodiment 16, wherein the 3-ketosteroid is selected among dehydrocholic acid (DHCA), the derivatives thereof such as, in particular, a salt, an amide or an alkyl ester of the acid. 18. Process as per embodiment 16 or 17, wherein the reduction takes place in the presence of and in particular with the consumption of NADPH and/or NADH. 19. Process as per embodiment 18, wherein consumed NADPH is regenerated by coupling with an NADPH-regenerating enzyme, wherein the latter is selected in particular among NADPH dehydrogenases, alcohol dehydrogenases (ADH) and NADPH-regenerating formate dehydrogenases (FDH) and an NADPH-regenerating glucose dehydrogenase (GDH), wherein the NADPH-regenerating enzyme is optionally expressed by a recombinant microorganism; and/or where consumed NADH is regenerated by coupling with an NADH-regenerating enzyme, wherein the latter is selected in particular among NADH-dehydrogenases, NADH-regenerating formate dehydrogenases (FDH), NADH-regenerating alcohol dehydrogenases (ADH), NADH-regenerating glucose-6-phosphate dehydrogenases (G6PDH), NADH-regenerating phosphite dehydrogenases (PtDH) and NADH-regenerating glucose dehydrogenases (GDH), wherein the NADH-regenerating enzyme is optionally expressed in a recombinant microorganism. 20. Process as per embodiment 19, wherein the NADPH-regenerating enzyme is selected among a. FDHs, including mutants of a NAD⁺-dependent FDH, which catalyzes at least the enzymatic oxidation of formic acid to CO₂, wherein the mutant in comparison with the unmutated enzyme additionally accepts NADP⁺ as cofactor; and b. GDHs. 21. Process for the preparation of ursodeoxycholic acid (UDCA) of the formula (1)

-   -   Error! Objects cannot be created from editing field codes.         in which         R represents alkyl, H, an alkali metal ion or N(R³)₄ ⁺, wherein         the radicals R³ are identical or different and represent H or         alkyl, or the group —CO₂R is replaced by the acid amide group         —CONR¹R², wherein R¹ and R² independently of one another         represent an alkyl radical;         in which         a) optionally a cholic acid (CA) of the formula (2)     -   Error! Objects cannot be created from editing field codes.         in which R has the abovementioned meanings or the group —CO₂R is         replaced by the acid amide group —CONR¹R² as defined hereinabove         is oxidized chemically to dehydrocholic acid (DHCA) of the         formula (3)     -   Error! Objects cannot be created from editing field codes.         in which R has the abovementioned meanings or the group —CO₂R is         replaced by the acid amide group —CONR¹R² as defined         hereinabove;         b) DHCA is reduced in the presence of at least one 3α-HSDH         mutant as per the definition in one of claims 1 to 8 (being         present as the isolated enzyme or expressed by a corresponding         recombinant microorganism) and in the presence of at least one         7β-HSDH (being present as the isolated enzyme or expressed by a         corresponding recombinant microorganism) to the corresponding         12-keto-ursodeoxycholic acid (12-keto UDCA) of the formula (5)     -   Error! Objects cannot be created from editing field codes.         in which R has the abovementioned meanings or the group —CO₂R is         replaced by the acid amide group —CONR¹R² as defined         hereinabove, in particular in the presence and with the         consumption of NADH and/or NADPH, and subsequently         c) 12-keto-UDCA of the formula (5) is reduced chemically to         UDCA; and         d) optionally, the reaction product is purified further.         22. Process as per embodiment 21, wherein at least step b) is         carried out in the presence of a recombinant microorganism as         per one of embodiments 12 to 15.         23. Process as per embodiment 21 or 22, wherein step b) is         coupled with identical or different cofactor regeneration         systems.         24. Process for the preparation of UDCA of the formula (1)     -   Error! Objects cannot be created from editing field codes.         in which         R represents alkyl, H, an alkali metal ion or N(R³)₄ ⁺, wherein         the radicals R³ are identical or different and represent H or         alkyl, or the group —CO₂R is replaced by the acid amide group         —CONR¹R², as defined hereinabove         in which         a) optionally a CA of the formula (2)     -   Error! Objects cannot be created from editing field codes.         in which R has the abovementioned meanings or the group —CO₂R is         replaced by the acid amide group —CONR¹R² as defined hereinabove         is oxidized chemically to DHCA of the formula (3)     -   Error! Objects cannot be created from editing field codes.         in which R has the abovementioned meanings or the group —CO₂R is         replaced by the acid amide group —CONR¹R² as defined         hereinabove;         b) DHCA is reduced in the presence of at least one 7β-HSDH and         in the presence of at least one 3α-HSDH to the corresponding         12-keto UDCA of the formula (5)     -   Error! Objects cannot be created from editing field codes.         in which R has the abovementioned meanings or the group —CO₂R is         replaced by the acid amide group —CONR¹R² as defined         hereinabove, in particular in the presence and with the         consumption of NADH and/or NADPH, and subsequently         c) 12-keto-UDCA of the formula (5) is reduced chemically to         UDCA; and         d) optionally, the reaction product is purified further;         wherein the conversions of step b) are carried out in the         presence of a recombinant microorganism as per one of         embodiments 12 to 15, such as, for example, in the presence of         whole cells of one or more different recombinant microorganisms         as per one of embodiments 12 to 15, wherein the microorganism(s)         includes the enzymes required for the conversion and the         cofactor regeneration in a manner described herein in greater         detail.         In this context, process step b) can be configured in different         ways. Either both enzymes (7β-HSDH mutant and 3α-HSDH) may be         present at the same time (for example one-top reaction with both         isolated enzymes, or one or more corresponding recombinant         microorganisms which express both enzymes are present), or the         partial reactions may proceed in any desired order (first the         7β-HSDH mutant-catalyzed reduction and then the         3α-HSDH-catalyzed reduction; or first the 3α-HSDH-catalyzed         reduction and then the 7β-HSDH mutant-catalyzed reduction).         Step b) may furthermore be coupled with a cofactor regeneration         system in which NADPH is regenerated by an NADPH regenerating         GDH with consumption of glucose, or is coupled with a cofactor         regeneration system in which consumed NADH is regenerated by an         NADH-regenerating GDH, ADH or FDH.         25. Bioreactor for carrying out a process as per one of         embodiments 16 to 24, in particular comprising at least one of         the enzymes 7β-HSDH, FDH and/or 3α-HSDH or their mutants; or         7β-HSDH, GDH and/or 3α-HSDH or their mutants.         The present invention is not limited to the concrete embodiments         described herein. Rather, the teaching of the present invention         allows a person skilled in the art to provide further         developments of the invention without undue burden. Thus, for         example, he may generate further enzyme mutants in a targeted         manner and screen and optimize them for the desired property         profile (improved cofactor dependency and/or stability, reduced         substrate inhibition); or he may isolate further suitable         wild-type enzymes (7β- and 3α-HSDHs, FDHs, GDHs ADHs etc.) and         use them in accordance with the invention. Furthermore, he may,         for example depending on the property profile (in particular         cofactor dependency) of the HSDHs used, such as, in particular,         7β-HSDH and 3α-HSDH or mutants thereof, select suitable         dehydrogenases which can be used for cofactor regeneration (GDH,         FHD, ADH and the like) and mutants thereof, and distribute the         selected enzymes to one or more expression constructs or vectors         and thus, if required, generate one or more recombinant         microorganisms which then make possible an optimized preparation         process based on whole cells.

Further Developments of the Invention

1. General definitions, and abbreviations used

Unless otherwise specified, the term “7β-HSDH” refers to a dehydrogenase enzyme which catalyzes at least the stereospecific and/or regiospecific reduction of DHCA or 7,12-diketo-3α-CA (7,12-diketo-LCA) to obtain 3,12-diketo-7β-CA or 12-keto-UDCA, in particular with the stoichiometric consumption of NADPH, and optionally the corresponding reverse reaction. In this context, the enzyme may be a native or a recombinantly produced enzyme. The enzyme may, in principle, be present as a mixture with cellular contaminants, such as, for example, protein contaminants, but preferably be present in pure form. Suitable detection methods are described, for example, in the experimental part which follows or are known from the literature (for example Characterization of NADP-dependent 7 beta-hydroxysteroid dehydrogenases from Peptostreptococcus productus and Eubacterium aerofaciens. S Hirano and N Masuda. Appl Environ Microbiol. 1982). Enzymes of this activity are classified under EC number 1.1.1.201. Suitable enzymes can be isolated for example from Colinsellea aerofaciens.

Unless otherwise specified, the term “3α-HSDH” refers to a dehydrogenase enzyme which catalyzes at least the stereospecific and/or regiospecific reduction of 3,12-diketo-7β-CA or DHCA to 12-keto-UDCA or 7,12-diketo-3α-CA (7,12-diketo-LCA), in particular with the stoichiometric consumption of NADH and/or NADPH, and optionally the corresponding reverse reaction. Suitable detection methods are described, for example, in the experimental part hereinbelow or are known from the literature. Suitable enzymes are obtainable for example from Comanomonas testosteroni (i.e. Comamonas testosteroni) (e.g. ATCC11996). An NADPH-dependent 3α-HSDH is known for example from rodents and can likewise be employed (Cloning and sequencing of the cDNA for rat liver 3 alpha-hydroxysteroid/dihydrodiol dehydrogenase, J E Pawlowski, M Huizinga and T M Penning, May 15, 1991 The Journal of Biological Chemistry, 266, 8820-8825). Enzymes of this activity are classified under EC number 1.1.1.50.

Unless otherwise specified, the term “GDH” refers to a dehydrogenase enzyme which catalyzes at least the oxidation of β-D-glucose to D-glucono-1,5-lactone with the stoichiometric consumption of NAD⁺ and/or NADP⁺ and optionally the corresponding reverse reaction. Suitable enzymes can be obtained for example from Bacillus subtilis or Bacillus megaterium. Enzymes of this activity are classified under EC number 1.1.1.47.

Unless otherwise specified, the term “FDH” refers to a dehydrogenase enzyme which catalyzes at least the oxidation of formic acid (or corresponding formate salts) to carbon dioxide with the stoichiometric consumption of NAD⁺ and/or NADP⁺, and optionally the corresponding reverse reaction. Suitable detection methods are described, for example, in the experimental part hereinbelow or from the literature. Suitable enzymes can be obtained for example from Candida boidinii, Pseudomonas sp or Mycobacterium vaccae. Enzymes with this activity are classified under EC number 1.2.1.2.

According to the invention, a “pure form” or a “pure” or “essentially pure” enzyme is understood according to the invention to be an enzyme with a degree of purity of more than 80, preferably more than 90, in particular more than 95 and especially more than 99% by weight, based on the total protein content, determined with the aid of customary protein detection methods such as, for example, the Biuret method or the protein detection as described by Lowry et al. (cf. description in R. K. Scopes, Protein Purification, Springer Verlag, New York, Heidelberg, Berlin (1982)).

A “redox equivalent” is understood as meaning a low-molecular-weight organic compound which can be used as an electron donor and/or electron acceptor, such as, for example, nicotinamide derivates such as NAD⁺ and NADH⁺, and their reduced forms NADH and NADPH, respectively. In the context of the present invention, “redox equivalent” and “cofactor” are used synonymously. For the purposes of the invention, therefore, a “cofactor” may also be paraphrased as a “redox-capable cofactor”, that is to say a cofactor which may be present in reduced and in oxidized form.

A “consumed” cofactor is understood as meaning the reduced or oxidized form of the cofactor which, in the course of a given reduction or oxidation reaction of a substrate, is converted into the corresponding oxidized or reduced form, respectively. Regeneration returns the oxidized or reduced cofactor formed during the reaction into its reduced or oxidized initial form, respectively, so that it is again available for the conversion of the substrate.

According to the present invention, an “altered cofactor utilization” is understood as meaning a qualitative or quantitative alteration in comparison with a reference. In particular, an altered cofactor utilization can be observed by carrying out amino acid sequence mutations. This alteration can then be discerned in comparison with the unmutated starting enzyme. Here, the activity relative to a certain cofactor may be increased or reduced by carrying out a mutation, or completely prevented. An altered cofactor utilization, however, also comprises alterations such that, instead of a specificity for a single cofactor, at least one further, second cofactor which is different from the first cofactor can now be utilized (i.e. extended cofactor utilization exists). Conversely, however, an ability, originally present, of utilizing two different cofactors may be altered such that specificity is increased for one of these cofactors only, or reduced or completely eliminated for one of these cofactors only. Thus, for example, an enzyme which is dependent on cofactor NAD (NADH) may, owing to an alteration of the cofactor utilization, now be dependent on both NAD (NADH) and on the cofactor NADP (NADPH), or the original dependency of NAD (NADH) may fully be converted into a dependency of NADP (NADPH), and vice versa.

Unless otherwise specified, the expressions “NAD⁺/NADH dependency” or “NADP⁺/NADPH dependency” should be interpreted broadly in accordance with the invention. These expressions comprise not only a “specific” dependencies, i.e. exclusively dependency on NAD⁺/NADH and/or NADP⁺/NADPH, but also the dependency of the enzymes used in accordance with the invention on both cofactors, i.e. dependency of NAD⁺/NADH and NADP⁺/NADPH.

The same applies to the expressions used “NAD⁺/NADH-accepting” and/or “NAD P+/NADPH-accepting”.

Unless otherwise specified, the expressions “NAD⁺/NADH regenerating” or “NADP⁺/NADPH regenerating” should be interpreted broadly in accordance with the invention. For these expressions comprise not only “specific”, i.e. exclusive ability of regenerating consumed cofactor NAD⁺/NADH and/or NADP⁺/NADPH, but also the ability of regenerating both cofactors, i.e. NAD⁺/NADH and NADP⁺/NADPH.

“Proteinogenic” amino acids comprise in particular (one-letter code): G, A, V, L, I, F, P, M, W, S, T, C, Y, N, Q, D, E, K, R and H.

According to the invention, an “immobilization” is understood as meaning the covalent or noncovalent bonding of a biocatalyst used in accordance with the invention, such as, for example, a 7β-HSDH, to a solid support material, i.e. a support material which is essentially insoluble in the surrounding liquid medium. According to the invention, whole cells, such as the recombinant microorganisms which are used in accordance with the invention, may also be immobilized with the aid of such supports.

A “substrate inhibition which is reduced in comparison with the unmutated enzymes” means that the substrate inhibition observed in the unmutated enzyme for a particular substrate can no longer be observed, i.e. is essentially no longer capable of being measured or commences only at a higher substrate concentration, i.e. the K_(i) value is increased.

N- or C-terminal extensions according to the invention comprise approximately 15 to 25 proteinogenic amino acid radicals which are attached to a terminal amino acid radical, optionally reversibly, for example via a protease cleavage site. Preferred residues comprise so-called “His tags” which are known per se, with, for example, 3, 6 or 9 consecutive histidine residues in the extension sequence.

According to the invention, a “cholic acid compound” is understood as meaning compounds with the carbon skeleton, in particular the steroid structure, of cholic acid, and the presence of keto and/or hydroxyl and/or acyloxy groups at ring position 7 and optionally positions 3 and/or 12.

A compound of a specific type such as, for example, a “cholic acid compound” or an “ursodeoxycholic acid compound” is, in particular, also understood as meaning derivatives of the underlying starting compound (such as, for example, cholic acid or ursodeoxycholic acid).

Such derivatives comprise “salts” such as, for example, alkali metal salts, such as lithium, sodium and potassium salts of the compounds; and ammonium salts, where under an ammonium salt is comprised the NH₄ ⁺ salt or those ammonium salts in which at least one hydrogen atom can be replaced by a C₁-C₆ alkyl radical. Typical alkyl radicals are, in particular X₁-C₄-alkyl radicals, such as methyl, ethyl, n- or i-propyl-, n-, sec- or tert-butyl, and n-pentyl and n-hexyl, and their analogs with one or more branches.

“Alkyl esters” of compounds according to the invention are, in particular, lower alkyl esters such as, for example, C₁-C₆-alkyl esters. Nonlimiting examples which may be mentioned are methyl, ethyl, n- or i-propyl-, n-, sec- or tert-butyl esters, or longer chain esters such as, for example, n-pentyl and n-hexyl esters, and their analogs with one or more branches.

“Amides” are, in particular, reaction products of acids according to the invention with ammonia or with primary or secondary monoamides. Such amides are, for example, mono- or di-C₁-C₆-alkyl monoamines, it being possible for the alkyl radicals independently of one another optionally to be substituted further such as, for example, by carboxyl, hydroxyl, halogen (such as F, Cl, Br, I), nitro and sulfonate groups.

“Acyl groups” according to the invention are, in particular, nonaromatic groups with 2 to 4 carbon atoms such as, for example, acetyl, propyonyl and butyryl, and aromatic groups with an optionally substituted mononuclear aromatic ring, suitable substitutents being selected for example among hydroxyl, halogen (such as F, Cl, Br, I), nitro and C₁-C₆-alkyl groups such as, for example, benzoyl or toluoyl.

The hydroxysteroid compounds applied and/or produced in accordance with the invention such as, for example, cholic acid, ursodeoxycholic acid, 12-keto-chenodeoxycholic acid, chenodeoxycholic acid and 7-keto-lithocholic acid, can be employed in the presence according to the invention, or obtained therefrom, in stereoisomerically pure form or in a mixture with other stereoisomers. Preferably, however, the compounds applied and/or produced are employed and/or isolated in essentially stereoisomerically pure form.

The structural formulae, their chemical names and the abbreviations of essential chemical compounds are tabulated in the table hereinbelow:

Formula Abbreviation Chemical name

CA Cholic acid

DHCA dehydrocholic acid

3,12-diketo-7β-CA 3,12-diketo-7β-cholanic acid

12-keto-UDCA 12-keto-ursodeoxycholic acid

UDCA Ursodeoxycholic acid

CA methyl ester Cholic acid methyl ester

3,7-diacetyl-CA methyl ester 3,7-diacetylcholic acid methyl ester*

12-keto-3,7-diacetyl-CA methyl ester 12-keto-3,7-diacetyl-cholanic acid methyl ester*

CDCA Chenodeoxycholic acid

7-keto-LCA 7-keto-lithocholic acid

7,12-diketo-LCA 7,12-diketo-lithocholic acid

12-keto-CDCA 12-keto-chenodeoxycholic acid

2. Proteins

The present invention is not restricted to the specifically disclosed proteins or enzymes with 7β-HSDH, FDH, GDH or 3α-HSDH activity and/or their mutants, but, rather, it also extends to functional equivalents thereof.

For the purposes of the present invention “functional equivalents” or analogs of the specifically disclosed enzymes are polypeptides which differ from the former and which still retain the desired biological activity such as, for example, 713 HSDH activity.

Thus, for example, the expression “functional equivalents” is understood as meaning enzymes which, in the 7β-HSDH, FDH, GDH or 3α-HSDH activity test used, have an activity which is by at least 1%, such as, for example, at least 10% or 20%, such as, for example, at least 50% or 75% or 90% higher or lower than that of a starting enzyme comprising an amino acid sequence defined herein.

Functional equivalents are furthermore stable preferably between pH 4 to 11 and advantageously have a pH optimum in a range of from pH 6 to 10, such as, in particular, 8.5 to 9.5, and a temperature optimum in the range of from 15° C. to 80° C. or 20° C. to 70° C., such as, for example, approximately 45 to 60° C. or approximately 50 to 55° C.

The 7β-HSDH activity may be detected with the aid of various known tests. Without being limited thereto, a test may be mentioned in which a reference substrate, such as, for example, CA or DHCA, is used under standardized conditions as defined in the experimental part.

Tests for determining the FDH, GDH, 7β-HSDH or 3α-HSDH activity are likewise known per se.

The expression “functional equivalents” is, according to the invention, in particular also to be understood as meaning “mutants” which, while having an amino acid in at least one sequence position of the abovementioned amino acid sequences which is different to the amino acid which has been mentioned specifically, retain one of the abovementioned biological activities. Therefore, “functional equivalents” comprise the mutants obtainable by one or more, such as, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15 amino acid additions, substitutions, deletions and/or inversions, it being possible for the abovementioned modifications to occur in any sequence position as long as they result in a mutant with the property profile according to the invention. Functional equivalents exists in particular also when the reactivity patterns of the mutant and the unmodified polypeptide agree in qualitative terms, i.e. when, for example, the same substrates are converted at different rates. Examples of suitable amino acid substitutions are compiled in the following table:

Original residue Examples of substitution Ala Ser Arg Lys Asn Gln; His Asp Glu Cys Ser Gln Asn Glu Asp Gly Pro His Asn; Gln Ile Leu; Val Leu Ile; Val Lys Arg; Gln; Glu Met Leu; Ile Phe Met; Leu; Tyr Ser Thr Thr Ser Trp Tyr Tyr Trp; Phe Val Ile; Leu

In “functional equivalents” in the above sense are also “precursors” of the above-described polypeptides and “functional derivatives” and “salts” of the polypeptides.

In this context, “precursors” are natural or synthetic precursors of the polypeptides with or without the desired biological activity.

The expression “salts” is understood as meaning not only salts of carboxyl groups, but also acid addition salts of amino groups of the protein molecules according to the invention. Salts of carboxyl groups can be prepared in a manner known per se and comprise inorganic salts such as, for example, sodium, calcium, ammonium, iron and zinc salts, and salts with organic bases such as, for example, amides, such as triethanolamine, arginine, lysine, piperidine and the like. Acid addition salts such as, for example, salts with mineral acids, such as hydrochloric acid or sulfuric acid, and salts with organic acids, such as acetic acid and oxalic acid, are likewise subject matter of the invention.

In “functional derivatives” of polypeptides according to the invention can also be generated at functional amino acid side-groups or at their N- or C-terminal end, using known techniques. Such derivatives comprise, for example, aliphatic esters of carboxyl groups, amides of carboxyl groups, obtainable by reaction with ammonia or with a primary or secondary amine; N-acyl derivates of free amino groups, prepared by reaction with acyl groups; or O-acyl derivatives free hydroxyl groups, prepared by reaction with acyl groups.

Naturally, “functional equivalents” also comprise polypeptides which are available from other organisms, and naturally occurring variants. For example, sequence comparisons may be used to determine regions of homologous sequence regions, and equivalent enzymes can be established on the basis of the specific requirements of the invention.

“Functional equivalents” likewise comprise fragments, preferably individual domains or sequence motifs, of the polypeptides according to the invention which have, for example, the desired biological function.

“Functional equivalents” are furthermore fusion proteins which have one of the abovementioned polypeptide sequences or functional equivalents derived therefrom and at least one other heterologous sequence functionally different therefrom in functional N- or C-terminal linkage (i.e. without significant mutual impairment of the functions of parts of the fusion proteins). Nonlimiting examples of such heterologous sequences are, for example, signal peptides, histidine anchors or enzymes.

“Functional equivalents” include in accordance with the invention homologs of the specifically disclosed proteins. These have at least 60%, preferably at least 75%, in particular at least 85%, such as, for example, 90, 91, 92, 93, 94, 95, 96, 97, 98 or 99%, homology (or identity) to one of the specifically disclosed amino acid sequences, calculated by the algorithm of Pearson and Lipman, Proc. Natl. Acad, Sci. (USA) 85(8), 1988, 2444-2448. A percentage homology or identity of a homologous polypeptide according to the invention means in particular percentage identity of the amino acid radicals based on the total length of one of the amino acid sequences which are described specifically herein.

The percentage identity data may also be determined by using BLAST alignments, the algorithm blastp (protein-protein BLAST), or by employing the Clustal settings specified hereinbelow.

In the event of possible protein glycosylation, “functional equivalents” according to the invention encompass proteins of the above-specified type in deglycosylated or glycosylated form, and modified forms obtainable by changing the glycosylation pattern.

Homologs of the proteins or polypeptides according to the invention can be generated by mutagenesis, for example by point mutation, or by extension or truncation of the protein.

Homologs of the proteins according to the invention can be identified by screening combinatorial libraries of mutants such as, for example, truncation mutants. For example, it is possible to generate a variegated library of protein variants by combinatorial mutagenesis at the nucleic acid level, such as, for example, by enzymatic ligation of a mixture of synthetic oligonucleotides. There exists a large number of processes which can be used to generate libraries of potential homologs from a degenerate oligonucleotide sequence. The chemical synthesis of a degenerate gene sequence may be carried out in an automatic DNA synthesizer, and the synthetic gene may then be ligated to a suitable expression vector. The use of a degenerate set of genes makes it possible to provide, in one mixture, all sequences which code for the desired set of potential protein sequences. Methods for synthesizing degenerate oligonucleotides are known to the person skilled in the art (for example Narang, S. A. (1983) Tetrahedron 39:3; Itakura et al. (1984) Annu. Rev. Biochem. 53:323; Itakura et al., (1984) Science 198:1056; Ike et al. (1983) Nucleic Acids Res. 11:477).

The prior art knows a variety of techniques for screening gene products of combinatorial libraries which have been generated by point mutations or truncation, and for screening cDNA libraries for gene products with a selected characteristic. These techniques can be adapted to the rapid screening of the gene libraries which have been generated by combinatorial mutagenesis of homologs according to the invention. The most frequently used techniques for screening large gene libraries undergoing high-throughput analysis comprise the cloning of the gene library into replicable expression vectors, transforming suitable cells with the resulting vector library and expressing the combinatorial genes under conditions under which the detection of the desired activity facilitates isolation of the vector which codes for the gene whose product has been detected. Recursive ensemble mutagenesis (REM), a technique which increases the frequency of functional mutants in the libraries, can be used in combination with the screening tests in order to identify homologs (Arkin and Yourvan (1992) PNAS 89:7811-7815; Delgrave et al. (1993) Protein Engineering 6(3):327-331).

The invention furthermore comprises the use of the 7β-HSDH wild type from Collinsella aerofaciens ATCC 25986 as is described in the applicant's WO 2011/064404, which is herewith expressly referred to.

This 7β-HSDH, which is obtainable from Collinsella aerofaciens DSM 3979, is characterized in particular by at least one further of the following properties, such as, for example, by 2, 3, 4, 5, 6 or 7 or all of such properties:

a) molecular weight (SDS gel electrophoresis): approximately 28-32 kDa, in particular approximately 29 to 31 kDa or approximately 30 kDa;

b) molecular weight (gel filtration under non-denaturing conditions, such as, in particular, without SDS): approximately 53 to 60 kDa, in particular approximately 55 to 57 kDa, such as 56.1 kDa. This confirms the dimeric nature of the Collinsella aerofaciens DSM 3979 7β-HSDH;

c) stereoselective reduction of the 7-carbonyl group of 7-keto-LCA to a 7β-hydroxy group;

d) pH optimum for the oxidation of UDCA in the range of from pH 8.5 to 10.5, in particular 9 to 10;

e) pH optimum for the reduction of DHCA and 7-keto-LCA in the range of from pH 3.5 to 6.5, in particular pH 4 to 6;

f) at least one kinetic parameter from the following table for at least one of the substrates/cofactors mentioned therein; in the range of from ±20%, in particular ±10%, ±5%, ±3%±2% or ±1% around the value mentioned in each case specifically in the table which follows.

K_(M) V_(max) k_(cat) (μM) (U/mg Protein)^(b)) (1 μmol/(μmol × min)) NADP⁺ 5.32 30.58 944.95 NADPH 4.50 33.44 1033.44 UDCA 6.23 38.17 1179.39 7-Keto-LCA 5.20 30.77 950.77 DHCA 9.23 28.33 875.35 NAD⁺  —^(a)) — Traces NADH — — Traces ^(a))no determination possible owing to the very low activity ^(b))1 U = 1 μmol/min

g) Phylogenetic sequence relationship of the prokaryotic Collinsella aerofaciens DSM 3979 7β-HSDH with the animal 11β-HSDH subgroup, comprising Cavia porcellus, Homo sapiens and Mus musulus.

For example, this 7β-HSDH displays the following properties or combinations of properties: a); b); a) and b); a) and/or b) and c); a) and/or b) and c) and d); a) and/or b) and c) and d) and e); a) and/or b) and c) and d) and e) and f).

Such a 7β-HSDH or functional equivalent derived therefrom is furthermore characterized by

-   a. the stereospecific reduction of a 7-ketosteroid to the     corresponding 7β-hydroxysteroid, and/or -   b. the regiospecific hydroxylation of a ketosteroid comprising a     keto group in the 7-position and at least one further keto group on     the steroid skeleton to give the corresponding 7β-hydroxysteroid,     such as, in particular, of dehydrocholic acid (DHCA) in the     7-position to give the corresponding 3,12-diketo-7β-cholanic acid,     catalyzes, and is for example NADPH-dependent.

Such a 7β-HSDH has, in particular, an amino acid sequence as per SEQ ID NO:2 (Accession NO: ZP_01773061) or a sequence derived therefrom with a degree of identity of at least 60%, such as, for example, 65, 70, 75, 80, 85 or 90, such as, for example, at least 91, 92, 93, 94, 95, 96, 97, 98, 99 or 99.5% to this sequence; optionally additionally characterized by one of the following properties or combinations of properties; a); b); a) and b); a) and/or b) and c); a) and/or b) and c) and d); a) and/or b) and c) and d) and e); a) and/or b) and c) and d) and e) and f) as per the above definition.

3. Nucleic Acids and Constructs 3. 1 Nucleic Acids

Subject-matter of the invention is also nucleic acid sequences which code for an enzyme with 7β-HSDH, FDH, GDH and/or 3α-HSDH activity and their mutants.

The present invention also relates to nucleic acids with a certain degree of identity to the specific sequences described herein.

The “identity” between two nucleic acids is understood as meaning the identity of the nucleotides over in each case the entire length of the nucleic acid, in particular the identity which is calculated by comparison with the aid of the Vector NTI Suite 7.1 software from Informax (USA) by applying the Clustal method (Higgins D G, Sharp P M. Fast and sensitive multiple sequence alignments on a microcomputer. Comput Appl. Biosci. 1989 April; 5(2):151-1), with the following parameter settings:

Multiple Alignment Parameters:

Gap opening penalty 10 Gap extension penalty 10 Gap separation penalty range  8 Gap separation penalty off % identity for alignment delay 40 M/55001-PCT Residue specific gaps off Hydrophilic residue gap off Transition weighing  0

Pairwise Alignment Parameter:

FAST algorithm on K-tuple size 1 Gap penalty 3 Window size 5 Number of best diagonals 5

Alternatively, the identity may also be determined by the method of Chenna, Ramu, Sugawara, Hideaki, Koike, Tadashi, Lopez, Rodrigo, Gibson, Toby J, Higgins, Desmond G, Thompson, Julie D. Multiple sequence alignment with the Clustal series of programs. (2003) Nucleic Acids Res 31 (13):3497-500, internet address: http://www.ebi.ac.uk/Tools/clustalw/index.html#, using the following parameters:

DNA Gap Open Penalty 15.0 DNA Gap Extension Penalty 6.66 DNA Matrix Identity Protein Gap Open Penalty 10.0 Protein Gap Extension Penalty 0.2 Protein matrix Gonnet Protein/DNA ENDGAP −1 Protein/DNA GAPDIST 4

All the nucleic acid sequences (single- and double-stranded DNA and RNA sequences such as, for example, cDNA and mRNA) mentioned herein can be generated in a manner known per se by chemical synthesis starting from the nucleotide units such as, for example, by fragment condensation of individual overlapping, complementary nucleic acid units of the double helix. Oligonucleotides may be synthesized chemically for example in a known manner, following the phosphoamidite method (Voet, Voet, 2^(nd) edition, Wiley Press New York, pages 896-897). The assembly of synthetic oligonucleotides and the filling-in of gaps with the aid of the DNA polymerase Klenow fragment and with the aid of ligation reactions and general cloning methods are described by Sambrook et al. (1989), Molecular Cloning: A laboratory manual, Cold Spring Harbor Laboratory Press.

Subject-matter of the invention is also nucleic acid sequences (single- and double-stranded DNA and RNA sequences, such as, for example, cDNA and mRNA) which code for any of the above polypeptides at their functional equivalents which may be prepared for example using artificial nucleotide analogs.

The invention relates both to isolated nucleic acid molecules which code for polypeptides or proteins according to the invention or biologically active sections thereof, and to nucleic acid fragments which may be used for example for use as hybridization probes or primers for identifying or amplifying coding nucleic acids according to the invention.

The nucleic acid molecules according to the invention may additionally comprise untranslated sequences from the 3′ and/or 5′ end of the coding gene region.

The invention furthermore comprises the nucleic acid molecules which are complementary to the specifically described nucleotide sequences or to a section thereof.

The nucleotide sequences according to the invention make possible the generation of probes and primers which may be used for identifying and/or cloning homologous sequences in other cell types and other organisms. Such probes or primers usually comprise a nucleotide sequence region which, under “stringent” conditions (see hereinbelow), hybridizes to at least approximately 12, preferably at least approximately 25, such as, for example, approximately 40, 50 or 75 consecutive nucleotides of a sense strand of a nucleic acid sequence according to the invention or of a corresponding antisense strand.

An “isolated” nucleic acid molecule is separated from other nucleic acid molecules which are present in the natural source of the nucleic acid and may additionally be essentially free of other cellular material or culture medium if it is generated by recombinant techniques, or free from chemical precursors or other chemicals if it is synthesized chemically.

A nucleic acid molecule according to the invention may be isolated by means of standard techniques of molecular biology and the sequence information provided in accordance with the invention. For example, cDNA may be isolated from a suitable cDNA library by using one of the specifically disclosed complete sequences or a section thereof as hybridization probe and using standard hybridization techniques (as described, for example, in Sambrook, J., Fritsch, E. F. and Maniatis, T. Molecular Cloning: A Laboratory Manual. 2^(nd) Edition, Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989). Moreover, a nucleic acid molecule comprising any of the disclosed sequences or a section thereof may be isolated by polymerase chain reaction, using the oligonucleotide primers which have been constructed on the basis of this sequence. The nucleic acid amplified in this manner may be cloned into a suitable vector and characterized by DNA sequence analysis. Furthermore, the oligonucleotides according to the invention may be generated by standard synthesis methods, for example using an automatic DNA synthesizer.

Nucleic acid sequences according to the invention or derivatives thereof, homologs or parts of these sequences may be isolated from other bacteria for example using customary hybridization methods or the PCR technique, for example by way of genomic libraries or cDNA libraries. These DNA sequences hybridize with the sequences according to the invention under standard conditions.

“Hybridizing” is understood as meaning the ability of a polynucleotide or oligonucleotide to bind, under standard conditions, to an almost complementary sequence, while unspecific bindings between non-complementary partners will not take place under these conditions. In this context, the sequences may be 90-100% complementary. The property of complementary sequences of being able to specifically bind to each other is exploited for example in the Northern or Southern blot technique or in the primer binding in PCR or RT-PCR.

For the hybridization, use is advantageously made of short oligonucleotides of the conserved regions. However, it is also possible to use longer fragments of the nucleic acids according to the invention, or the complete sequences, for the hybridization. Depending on the nucleic acid employed (oligonucleotide, longer fragment or complete sequence) or depending on which nucleic acid type, DNA or RNA, is used for the hybridization, these standard conditions will vary. Thus, for example, the melting temperatures for DNA:DNA hybrids are approximately 10° C. lower than those of DNA:RNA hybrids of the same length.

Depending on the nucleic acid, standard conditions are understood as meaning, for example, temperatures of between 42 and 58° C. in an aqueous buffer solution having a concentration of between 0.1 to 5×SSC (1×SSC=0.15 M NaCl, 15 mM sodium citrate, pH 7.2) or additionally in the presence of 50% formamide, for example 42° C. in 5×SSC, 50% formamide. Advantageously, the hybridization conditions for DNA:DNA hybrids are 0.1×SSC and temperatures between about 20° C. to 45° C., preferably between about 30° C. to 45° C. For DNA:RNA hybrids, the hybridization conditions are advantageously 0.1×SSC and temperatures of between about 30° C. to 55° C., preferably between about 45° C. to 55° C. These temperatures stated for the hybridization are melting temperature values which have been calculated by way of example for a nucleic acid having a length of approximately 100 nucleotides and a G+C content of 50% in the absence of formamide. The experimental conditions for the DNA hybridization are described in specialist textbooks of genetics, such as, for example, Sambrook et al., “Molecular Cloning”, Cold Spring Harbor Laboratory, 1989, and may be calculated using formulae known to the person skilled in the art, for example as a function of the length of the nucleic acids, the type of hybrids or the G+C content. The person skilled in the art may obtain further information with regard to hybridization from the following textbooks: Ausubel et al. (eds.), 1985, Current Protocols in Molecular Biology, John Wiley & Sons, New York; Hames and Higgins (eds.), 1985, Nucleic Acids Hybridization: A Practical Approach, IRL Press at Oxford University Press, Oxford; Brown (ed.), 1991, Essential Molecular Biology: A Practical Approach, IRL Press at Oxford University Press, Oxford.

The “hybridization” may take place in particular under stringent conditions. Such hybridization conditions are described for example in Sambrook, J., Fritsch, E. F., Maniatis, T., in: Molecular Cloning (A Laboratory Manual), 2nd Edition, Cold Spring Harbor Laboratory Press, 1989, pages 9.31-9.57 or in Current Protocols in Molecular Biology, John Wiley & Sons, N.Y. (1989), 6.3.1-6.3.6.

“Stringent” hybridization conditions are understood as meaning in particular the following: incubation overnight at 42° C. in a solution composed of 50% formamide, 5×SSC (750 mM NaCl, 75 mM trisodium citrate), 50 mM sodium phosphate (pH 7.6), 5×Denhardt's solution, 10% dextran sulfate and 20 g/ml denatured sheared salmon sperm DNA, followed by a wash step of the filters with 0.1×SSC at 65° C.

Subject matter of the invention are also derivatives of the specifically disclosed or derivable nucleic acid sequences.

Thus, further nucleic acid sequences according to the invention may be derived from, for example, SEQ ID NO:1, 3 or 10 and differ therefrom by addition, substitution, insertion or deletion of individual or several nucleotides, but still code for polypeptides with the desired property profile.

Also comprised by the invention are those nucleic acid sequences which comprise “silent” mutations or which are altered, as compared with a specifically mentioned sequence, according to the codon usage of a specific source organism or host organism, as are naturally occurring variants thereof, such as, for example, splice variants or allelic variants.

Another subject matter is sequences obtainable by way of conservative nucleotide substitutions (i.e. the amino acid in question is replaced with an amino acid of the same charge, size, polarity and/or solubility).

Subject matter of the invention is also the molecules which are derived from the specifically disclosed nucleic acids by way of sequence polymorphisms. These genetic polymorphisms may exist between individuals within a population as a result of natural variation. These natural variations usually give rise to a variance of from 1 to 5% in the nucleotide sequence of a gene.

Derivatives of nucleic acid sequences according to the invention and which have the sequence SEQ ID NO:1, 3 or 10 are understood as meaning, for example, allelic variants which, at the deduced amino acid level, have at least 60% homology, preferably at least 80% homology, very especially preferably at least 90% homology over the entire sequence region (in respect of homology at the amino acid level, the reader may refer to the above comments regarding the polypeptides). Advantageously, the homologies may be higher across subregions of the sequences.

Furthermore, derivatives are also understood as meaning homologs of the nucleic acid sequences according to the invention, in particular of the SEQ ID NO:1, 3 or 10 for example fungal or bacterial homologs, truncated sequences, single-stranded DNA or RNA of the coding and noncoding DNA sequence. Thus, for example homologs to the SEQ ID NO:1, 3 or 10 have a homology of at least 40%, preferably of at least 60%, especially preferably of at least 70%, very especially preferably of at least 80% homology at the DNA level over the entire DNA region specified in SEQ ID No:1, 3 or 10.

Furthermore, derivatives are understood as meaning for example fusions with promoters. The promoters which are located upstream of the nucleotide sequences indicated may have been altered by at least one nucleotide substitution, at least one insertion, inversion and/or deletion, without, however, the functionality and efficacy of the promoters being adversely affected. Furthermore, the efficacy of the promoters may be increased by altering their sequence, or the promoters may be replaced entirely with more active promoters, including promoters from organisms of other species.

In addition, a person skilled in the art is familiar with processes for generating functional mutants.

Depending on the technique used, a person skilled in the art may introduce completely random or, also more specific mutations into genes or also non-coding nucleic acid regions (for example regions which are important for regulating expression) and subsequently generate gene libraries. The molecular-biological methods required for this purpose are known to a person skilled in the art and described, for example, in Sambrook and Russell, Molecular Cloning. 3^(rd) Edition, Cold Spring Harbor Laboratory Press 2001.

Methods for modifying genes and thus for altering the protein coded by them have been familiar to a person skilled in the art for a long time, such as, for example,

-   -   site-specific mutagenesis, where single or multiple nucleotides         of a gene are substituted specifically (Trower M K (Ed.) 1996;         In vitro mutagenesis protocols. Humana Press, New Jersey),     -   saturation mutagenesis, in which a codon for any desired amino         acid may be substituted or added at any desired position of a         gene (Kegler-Ebo D M, Docktor C M, DiMaio D (1994) Nucleic Acids         Res. 22:1593; Barettino D, Feigenbutz M, Valcàrel R, Stunnenberg         H G (1994) Nucleic Acids Res 22:541; Barik S (1995) Mol.         Biotechnol. 3:1),     -   error-prone polymerase chain reaction (PCR), in which nucleotide         sequences are mutated by defectively operating DNA polymerases         (Eckert K A, Kunkel T A (1990) Nucleic Acids Res. 18:3739);     -   the passaging of genes in mutator strains in which for example         on account of deficient DNA repair mechanisms the mutation rate         of nucleotide sequences is increased (Greener A, Callahan M,         Jerpseth B (1996) An efficient random mutagenesis technique         using an E. coli mutator strain. In: Trower M K (Ed.) In vitro         mutagenesis protocols. Humana Press, New Jersey), or     -   DNA shuffling, in which a pool of closely related genes is         formed and digested, and the fragments are used as templates for         a polymerase chain reaction, in which full-length mosaic genes         are ultimately generated by repeated strand separation and         reannealing (Stemmer W P C (1994) Nature 370:389; Stemmer W P         C (1994) Proc. Natl. Acad. Sci. USA 91:10747).

Employing what is known as directed evolution (described, inter alia, in Reetz M T and Jaeger K-E (1999), Topics Curr Chem 200:31; Zhao H, Moore J C, Volkov A A, Arnold F H (1999), Methods for optimizing industrial enzymes by directed evolution, In: Demain A L, Davies J E (Ed.) Manual of industrial microbiology and biotechnology. American Society for Microbiology), a person skilled in the art can also generate functional mutants in a targeted manner, also on a large scale. Here, gene libraries of the respective proteins are generated in a first step, and these gene libraries may be set up employing, for example, the methods specified hereinabove. The gene libraries are expressed in a suitable manner, for example by bacteria or by phage display systems.

The relevant genes of host organisms which express functional mutants with properties which correspond largely to the desired properties may be subjected to a further mutation cycle. The steps of mutation and of selection or of screening may be repeated iteratively until the functional mutants which are present show the desired properties to a sufficient extent. A limited number of mutations such as, for example, 1 to 5 mutations may be generated stepwise by this iterative approach, and their influence on the relevant enzyme property may be assessed and selected. The selected mutant may then be subjected in the same manner to a further mutation step. This allows the number of individual mutants to be studied to be reduced significantly.

The results according to the invention also provide important information in respect of structure and sequence of the enzymes in question, which enzymes are required for generating, in a targeted fashion, further enzymes with desired modified properties. In particular, it is possible to define what are known as “hot spots”, i.e. sequence sections which are potentially suitable for modifying an enzyme property via the introduction of targeted mutations.

3.2 Constructs

Subject matter of the invention is furthermore expression constructs, comprising, under the genetic control of regulatory nucleic acid sequences, a nucleic acid sequence which encodes at least one polypeptide according to the invention; and vectors comprising at least one of these expression constructs.

An “expression unit” is understood as meaning, according to the invention, a nucleic acid with expression activity, which comprises a promoter as defined herein and which, after functional linkage to a nucleic acid to be expressed or to a gene, regulates the expression, that is to say the transcription and the translation, of said nucleic acid or said gene. This is why, in this context, an expression unit is also referred to as a “regulatory nucleic acid sequence”. Further regulatory elements, such as, for example, enhancers, may be present in addition to the promoter.

According to the invention, an “expression cassette” or “expression construct” is understood as meaning an expression unit which is functionally linked to the nucleic acid to be expressed or the gene to be expressed. In contrast to an expression unit, therefore, an expression cassette not only comprises nucleic acid sequences which regulate transcription and translation, but also the nucleic acid sequences which are to be expressed as protein as a consequence of the transcription and translation.

In the context of the invention, the terms “expression” or “overexpression” describe the production or increase of the intracellular activity of one or more enzymes in a microorganism, which enzymes are coded by the corresponding DNA. To this end, for example, a gene may be introduced into an organism, an existing gene may be replaced by a different gene, the copy number of the gene(s) may be increased, a strong promoter may be used, or a gene may be used which encodes a corresponding enzyme with a high activity, and, optionally, these measures can be combined.

Preferably, such constructs according to the invention comprise a promoter upstream, i.e. at the 5′ end of the particular coding sequence, and a terminator sequence downstream, i.e. at the 3′ end, and, optionally, further customary regulatory elements, in each case operably linked to the coding sequence.

A “promoter”, a “nucleic acid with promoter activity” or a “promoter sequence” is understood as meaning in accordance with the invention a nucleic acid which, in functional linkage to a nucleic acid to be transcribed, regulates the transcription of said nucleic acid.

In this context, a “functional” or “operable” linkage is understood as meaning, for example, the sequential arrangement of one of the nucleic acids with promoter activity and a nucleic acid sequence to be transcribed and, optionally, further regulatory elements such as, for example, nucleic acid sequences which ensure the transcription of nucleic acids, and, for example, a terminator in such a way that each of the regulatory elements is able to carry out its function as intended in the transcription of the nucleic acid sequence. A direct linkage in the chemical sense is not mandatory in this context. Genetic control sequences, such as, for example, enhancer sequences, may also exert their function on the target sequence from positions which are further removed, or even from different DNA molecules. Preferred arrangements are those in which the nucleic acid sequence to be transcribed is positioned after the promoter sequence (i.e. at its 3′ end) so that the two sequences are covalently linked to each other. In this context, the distance between the promoter sequence and the nucleic acid sequence to be expressed recombinantly may be less than 200 base pairs or less than 100 base pairs or less than 50 base pairs.

Examples of further regulatory elements which may be mentioned besides promoters and terminator are targeting sequences, enhancers, polyadenylation signals, selectable markers, amplification signals, replication origins and the like. Suitable regulatory sequences are described, for example, in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif. (1990).

Nucleic acid constructs according to the invention comprise in particular a sequence SEQ ID NO:1, 3 or 10 or derivatives and homologs thereof, and the nucleic acid sequences which can be derived therefrom and which have been operably or functionally linked to one or more regulatory signals, advantageously for controlling, for example increasing, gene expression.

In addition to these regulatory sequences, the natural regulation of these sequences may still be present upstream of the actual structural genes and may optionally have been genetically altered in such a way that the natural regulation has been switched off and the expression of the genes has been increased. However, the nucleic acid construct may also have a simpler design, i.e. no additional regulatory signals have been inserted upstream of the coding sequence, and the natural promoter, together with its regulation, has not been removed. Instead, the natural regulatory sequence is mutated in such a way that regulation no longer takes place, and gene expression is increased.

A preferred nucleic acid construct advantageously also comprises one or more of the previously mentioned “enhancer” sequences which are functionally linked to the promoter and which enable expression of the nucleic acid sequence to be increased. Additional advantageous sequences, such as further regulatory elements or terminators, may also be inserted at the 3′ end of the DNA sequences. The nucleic acids according to the invention may be present in the construct in one or more copies. The construct may additionally comprise further markers such as antibiotic resistances or auxotrophy-complementing genes, optionally for the purpose of selecting the construct.

Examples of suitable regulatory sequences are present in promoters such as cos, tac, trp, tet, trp-tet, lpp, lac, lpp-lac, lacl^(q), T7, T5, T3, gal, trc, ara, rhaP (rhaP_(BAD))SP6, lambda-P_(R) or in the lambda-P_(L) promotors which are advantageously used in Gram-negative bacteria. Other advantageous regulatory sequences are present for example in the Gram-positive promoters amy and SPO2, in the yeast or fungal promoters ADC1, MFalpha, AC, P-60, CYC1, GAPDH, TEF, rp28, ADH. Artificial promoters may also be used for regulation purposes.

For the purposes of expression in a host organism, the nucleic acid construct is advantageously inserted into a vector such as, for example, a plasmid or a phage, which enables the genes to be expressed optimally in the host. Vectors, in addition to plasmids and phages, are also understood as meaning all the other vectors known to a person skilled in the art, i.e. for example viruses such as SV40, CMV, baculovirus and adenovirus, transposons, IS elements, phasmids, cosmids and linear or circular DNA. These vectors may be replicated autonomously in the host organism or replicated chromosomally. These vectors constitute a further development of the invention.

Examples of suitable plasmids are, for example, in E. coli pLG338, pACYC184, pBR322, pUC18, pUC19, pKC30, pRep4, pHS1, pKK223-3, pDHE19.2, pHS2, pPLc236, pMBL24, pLG200, pUR290, pIN-III¹¹³-B1, λgt11 or pBdCl, in Streptomyces pIJ101, pIJ364, pIJ702 or pIJ361, in Bacillus pUB110, pC194 or pBD214, in Corynebacterium pSA77 or pAJ667, in fungi pALS1, pIL2 or pBB116, in yeasts 2alphaM, pAG-1, YEp6, YEp13 or pEMBLYe23 or in plants pLGV23, pGHlac⁺, pBIN19, pAK2004 or pDH51. The plasmids mentioned are a small selection of the possible plasmids. Other plasmids are well known to a person skilled in the art and can be found, for example, in the book Cloning Vectors (Eds. Pouwels P. H. et al. Elsevier, Amsterdam-New York-Oxford, 1985, ISBN 0 444 904018).

In a further embodiment of the vector, the vector comprising the nucleic acid construct according to the invention or the nucleic acid according to the invention may also advantageously be introduced into the microorganisms in the form of a linear DNA and integrated into the genome of the host organism via heterologous or homologous recombination. This linear DNA may consist of a linearized vector, such as a plasmid, or only of the nucleic acid construct or of the nucleic acid according to the invention.

In order to express heterologous genes optimally in organisms, it is advantageous to alter the nucleic acid sequences in accordance with the specific “codon usage” employed in the organism. The “codon usage” can be determined readily with the aid of computer analyses of other known genes of the organism in question.

An expression cassette according to the invention is generated by fusing a suitable promoter to a suitable coding nucleotide sequence and to a terminator signal or polyadenylation signal. Common recombination and cloning techniques are used for this purpose, as are described, for example, in T. Maniatis, E. F. Fritsch and J. Sambrook, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1989) and in T. J. Silhavy, M. L. Berman and L. W. Enquist, Experiments with Gene Fusions, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1984) and in Ausubel, F. M. et al., Current Protocols in Molecular Biology, Greene Publishing Assoc. and Wiley Interscience (1987).

For expression purposes in a suitable host organism, the recombinant nucleic acid construct or gene construct is advantageously inserted into a host-specific vector which enables the genes to be expressed optimally in the host. Vectors are well known to a person skilled in the art and can be found for example in “Cloning Vectors” (Pouwels P. H. et al., Eds., Elsevier, Amsterdam-New York-Oxford, 1985).

4 Microorganisms

Depending on the context, the term “microorganism” may be understood as meaning the starting (wild-type) microorganism or a genetically modified recombinant microorganism, or both.

It is possible to prepare, with the aid of the vectors according to the invention, recombinant microorganisms which are transformed for example with at least one vector according to the invention and which may be used for producing the polypeptides according to the invention. Advantageously, the above-described recombinant constructs according to the invention are introduced into a suitable host system and expressed therein. In this connection, familiar cloning and transfection methods with which a person skilled in the art is familiar, such as, for example, coprecipitation, protoplast fusion, electroporation, retroviral transfection and the like, are preferably used in order to cause said nucleic acids to be expressed in the particular expression system. Suitable systems are described, for example, in Current Protocols in Molecular Biology, F. Ausubel et al., Eds., Wiley Interscience, New York 1997, or Sambrook et al. Molecular Cloning: A Laboratory Manual. 2^(nd) edition, Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989. An overview of bacterial expression systems for the heterologous expression of proteins is also provided, for example, by Terpe, K. Appl. Microbiol. Biotechnol. (2006) 72: 211-222.

Recombinant host organisms for the nucleic acid according to the invention or the nucleic acid construct are, in principle, all prokaryotic or eukaryotic organisms. It is advantageous to employ, as host organisms, microorganisms such as bacteria, fungi or yeasts. It is advantageous to employ Gram-positive or Gram-negative bacteria, preferably bacteria of the families Enterobacteriaceae, Pseudomonadaceae, Rhizobiaceae, Streptomycetaceae or Nocardiaceae, especially preferably bacteria of the genera Escherichia, Pseudomonas, Comamonas, Streptomyces, Nocardia, Burkholderia, Salmonella, Agrobacterium, Clostridium or Rhodococcus. Very especially preferred is the genus and species Escherichia coli. Further advantageous bacteria can furthermore be found in the group of alpha-proteobacteria, beta-proteobacteria or gamma-proteobacteria.

In this context, the host organism(s) according to the invention preferably comprise(s) at least one of the nucleic acid sequences, nucleic acid constructs or vectors which are described in this invention and which encode an enzyme with 7β-HSDH activity as defined hereinabove.

Depending on the host organism, the organisms used in the process according to the invention are grown, or cultured, in a manner known to a person skilled in the art. As a rule, microorganisms are grown in a liquid medium comprising a carbon source, usually in the form of sugars, a nitrogen source, usually in the form of organic nitrogen sources such as yeast extract or salts such as ammonium sulfate, trace elements such as salts of iron, manganese and magnesium and optionally vitamins at temperatures of between 0° C. and 100° C., preferably between 10° C. to 60° C., while passing in oxygen. The pH of the liquid medium can either be kept constant, that is to say regulated during the culturing, or not. The culturing may take place batchwise, semibatchwise or continuously. Nutrients may be provided at the beginning of the fermentation or fed in semicontinuously or continuously.

5. Production of UDCA

Step 1: Chemical Conversion of CA into DHCA

The hydroxyl groups of CA are oxidized to carbonyl groups in a manner known per se via the traditional chemical route, using chromic acids or chromates in acidic solution (for example H₂SO₄). This gives rise to DHCA.

Step 2: Enzymatic or Microbial Conversion of DHCA into 12-Keto-UDCA

DHCA is reduced specifically in aqueous solution by 3α-HSDH and 7β-HSDH or mutants thereof in the presence of NADPH or NADH to give 12-keto-UDCA. The cofactor NADPH or NADH can be regenerated by an ADH or FDH or GDH or mutants thereof from isopropanol or sodium formate or glucose, respectively. The reaction proceeds under mild conditions. For example, the reaction may be carried out at pH=6 to 9, in particular approximately pH=8 and at approximately 10 to 30, 15 to 25 or approximately 23° C.

In the event of a microbial conversion step, recombinant microorganisms which express the enzyme activity/activities required may be cultured anaerobically or aerobically in suitable liquid media in the presence of the substrate to be converted (DHCA). Suitable culturing conditions are known per se to a person skilled in the art. They comprise conversions in the pH range of, for example, 5 to 10 or 6 to 9, at temperatures in the range of from 10 to 60 or 15 to 45 or 25 to 40 or 37° C. Suitable media comprise for example the LB and TB media described hereinbelow. In this context, the conversion time may for example be carried out batchwise or continuously or in any other customary process variant (as described hereinabove). The conversion time may, in this context, be for example in the range of from minutes to several hours or days and may amount to, for example, 1 h to 48 h. Optionally, if enzymatic activity is not expressed continuously, the latter may be initiated by adding a suitable inductor after a target cell density of, for example, approximately OD₆₀₀=0.5 to 1.0 has been reached.

As regards the operation of the fermentation, additions to the medium, enzyme immobilization and isolation of the substances of value, further suitable modifications of the microbial production process which are possible can also be found in the following section regarding “production of the enzymes or mutants”.

Step 3: Chemical Conversion of 12-Keto-UDCA to UDCA

The 12-carbonyl group of 12-keto-UDCA is removed in a manner known per se by means of Wolff-Kischner reduction, whereby UDCA is generated from 12-keto-UDCA. In the reaction, the carbonyl is first reacted with hydrazine to give the hydrazone. Thereafter, the hydrazone is heated to 200° C. in the presence of a base (for example KOH); during this process, nitrogen is eliminated, giving rise to UDCA.

6. Recombinant Production of the Enzymes and Mutants

Subject matter of the invention is furthermore processes for the recombinant production of polypeptides according to the invention or functional biologically active fragments thereof, wherein a polypeptide-producing microorganism is cultured, the expression of the polypeptides is optionally induced and the polypeptides are isolated from the culture. If desired, the polypeptides may, in this manner, also be produced on an industrial scale.

The microorganisms which have been produced in accordance with the invention can be grown continuously or discontinuously by the batch method, the fed-batch method or the repeated fed-batch method. An overview of known culture methods can be found in the textbook by Chmiel (Bioprozeβtechnik 1. Einführung in die Bioverfahrenstechnik [Bioprocess Engineering 1. Introduction to Bioprocess Technology] (Gustav Fischer Verlag, Stuttgart, 1991)) or in the textbook by Storhas (Bioreaktoren and periphere Einrichtungen [Bioreactors and Peripheral Units] (Vieweg Verlag, Brunswick/Wiesbaden, 1994)).

The culture medium to be used must suitably meet the needs of the strains in question. Descriptions of culture media for a variety of microorganisms are found in the manual “Manual of Methods for General Bacteriology” of the American Society for Bacteriology (Washington D. C., USA, 1981).

These media which can be employed in accordance with the invention usually encompass one or more carbon sources, nitrogen sources, inorganic salts, vitamins and/or trace elements.

Preferred carbon sources are sugars such as mono-, di- or polysaccharides. Examples of very good carbon sources are glucose, fructose, mannose, galactose, ribose, sorbose, ribulose, lactose, maltose, sucrose, raffinose, starch or cellulose. Sugars may also be added to the media via complex compounds, such as molasses, or other by-products of sugar refining. It may also be advantageous to add mixtures of various carbon sources. Other possible carbon sources are oils and fats such as, for example, soya oil, sunflower oil, peanut oil and coconut fat, fatty acids such as, for example, palmitic acid, stearic acid or linoleic acid, alcohols such as, for example, glycerol, methanol or ethanol, and organic acids such as, for example, acetic acid or lactic acid.

Nitrogen sources are usually organic or inorganic nitrogen compounds or materials comprising these compounds. Examples of nitrogen sources encompass ammonia gas or ammonium salts such as ammonium sulfate, ammonium chloride, ammonium phosphate, ammonium carbonate or ammonium nitrate, nitrates, urea, amino acids or complex nitrogen sources such as corn steep liquor, soya meal, soya protein, yeast extract, meat extract and others. The nitrogen sources can be used individually or as a mixture.

Inorganic salt compounds which may be present in the media encompass the chloride, phosphorus or sulfate salts of calcium, magnesium, sodium, cobalt, molybdenum, potassium, manganese, zinc, copper and iron.

Inorganic sulfur-containing compounds such as, for example, sulfates, sulfites, dithionites, tetrathionates, thiosulfates, sulfides, but also organic sulfur compounds, such as mercaptans and thiols, may be used as the sulfur source.

Phosphoric acid, potassium dihydrogenphosphate or dipotassium hydrogenphosphate or the corresponding sodium-containing salts may be used as phosphorus source.

Sequestrants may be added to the medium in order to maintain the metal ions in solution. Particularly suitable sequestrants encompass dihydroxyphenols, such as catechol or protocatechuate, or organic acids such as citric acid.

Usually, the fermentation media employed in accordance with the invention also comprise other growth factors such as vitamins or growth promoters, which include, for example, biotin, riboflavin, thiamine, folic acid, nicotinic acid, panthothenate and pyridoxine. Growth factors and salts are frequently obtained from complex media components such as yeast extract, molasses, corn steep liquor and the like. Moreover, suitable precursors may be added to the culture medium. The exact composition of the compounds in the media depends greatly on the experiment in question and is decided individually for each individual case. Information on the optimization of media can be found in the textbook “Applied Microbiol. Physiology, A Practical Approach” (Eds P. M. Rhodes, P. F. Stanbury, IRL Press (1997) pp. 53-73, ISBN 0 19 963577 3). Growth media can also be obtained from commercial sources, such as Standard 1 (Merck) or BHI (brain heart infusion, DIFCO) and the like.

All of the media components are sterilized, either by means of heat (20 minutes at 1.5 bar and 121° C.) or by filter sterilization. The components may be sterilized either together or, if necessary, separately. All of the media components may be present at the beginning of the culturing or else be added continuously or batchwise, as desired.

The culture temperature is normally between 15° C. and 45° C., preferably 25° C. to 40° C., and can be kept constant during the experiment or else be varied. The pH of the medium should be in the range of from 5 to 8.5, preferably around 7.0. The pH for the culturing can be controlled during culturing by addition of basic compounds such as sodium hydroxide, potassium hydroxide, ammonia or ammonia water, or acidic compounds such as phosphoric acid or sulfuric acid. Antifoam agents such as, for example, fatty acid polyglycol esters may be employed to control foam development. To maintain plasmid stability, suitable selectively acting substances such as, for example, antibiotics may be added to the medium. To maintain aerobic conditions, oxygen or oxygen-containing gas mixtures, for example ambient air, are passed into the culture. The culture temperature is normally 20° C. to 45° C. and. The culture is continued until a maximum of the desired product has formed. This aim is normally achieved within 10 hours to 160 hours.

Thereafter, the fermentation broth is processed further. Depending on what is required, all or some of the biomass may be removed from the fermentation broth by separation methods such as, for example, centrifugation, filtration, decanting or a combination of these methods, or else be left completely in said broth.

If the polypeptides are not secreted into the culture medium, the cells may also be disrupted and the product may be obtained from the lysate by known protein isolation methods. The cells can optionally be disrupted by high-frequency ultrasound, by high pressure such as, for example, in a French pressure cell, by osmolysis, by the action of detergents, lytic enzymes or organic solvents, by homogenizers or by a combination of several of the abovementioned methods.

The polypeptides may be purified using known chromatographic techniques, such as molecular sieve chromatography (gel filtration), such as Q-Sepharose chromatography, ion exchange chromatography and hydrophobic chromatography, and also using other common methods such as ultrafiltration, crystallization, salting out, dialysis and native gel electrophoresis. Suitable methods are described for example in Cooper, F. G., Biochemische Arbeitsmethoden [Biochemical working methods], Verlag Walter de Gruyter, Berlin, N.Y. or in Scopes, R., Protein Purification, Springer Verlag, New York, Heidelberg, Berlin.

To isolate the recombinant protein, it may be advantageous to use vector systems or oligonucleotides which extend the cDNA by specific nucleotide sequences and thus encode modified polypeptides or fusion proteins which serve, for example, the purpose of simpler purification. Such modifications which are suitable are, for example, what are known as “tags”, which act as anchors, such as, for example, the modification known as hexa-histidine anchor or epitopes capable of being recognized by antibodies as being antigens (described, for example, in Harlow, E. and Lane, D., 1988, Antibodies: A Laboratory Manual. Cold Spring Harbor (N.Y.) Press). These anchors may serve to attach the proteins to a solid support, such as, for example, a polymer matrix which may be packed into, for example, a chromatography column, or for attaching the proteins to a microtiter plate or any other support.

At the same time, these anchors can also be used for the identification of the proteins. For identifying the proteins, customary markers, such as fluorescent dyes, enzyme markers that, after reaction with a substrate, form a detectable reaction product, or radioactive markers, can moreover be used alone or in combination with the anchors for derivatization of the proteins.

7. Enzyme Immobilization

In the methods described herein, the enzymes according to the invention may be employed in free or immobilized form. An immobilized enzyme is understood as meaning an enzyme which is fixed to an inert support. Suitable support materials, and the enzymes immobilized thereon, are known from EP-A-1149849, EP-A-1 069 183 and DE-OS 100193773 and from the literature cited therein. In this respect, the disclosure of said documents is referred to in its entirety. The suitable support materials include, for example, clays, clay minerals such as kaolinite, diatomaceous earth, perlite, silica, alumina, sodium carbonate, calcium carbonate, cellulose powder, anion exchange materials, synthetic polymers such as polystyrene, acrylic resins, phenol-formaldehyde resins, polyurethanes and polyolefins such as polyethylene and polypropylene. The support materials are conventionally employed in a finely divided, particulate form for the preparation of the supported enzymes, with porous forms being preferred. The particle size of the support material is conventionally not more than 5 mm, in particular not more than 2 mm (grading curve). Analogously, on use of the dehydrogenase as a whole-cell catalyst, a free or an immobilized form may be chosen. Support materials are, for example, calcium alginate and carrageenan. Enzymes as well as cells may also be crosslinked directly using glutaraldehyde (crosslinking to CLEAs). Corresponding and further immobilization processes are described, for example, in J. Lalonde and A. Margolin “Immobilization of Enzymes” in K. Drauz and H. Waldmann, Enzyme Catalysis in Organic Synthesis 2002, Vol. III, 991-1032, Wiley-VCH, Weinheim.

Experimental Part:

Unless otherwise specified, the cloning steps carried out within the scope of the present invention, such as, for example, restriction cleavages, agarose gel electrophoresis, the purification of DNA fragments, the transfer of nucleic acids to nitrocellulose and nylon membranes, the linking of DNA fragments, the transformation of microorganisms, the culturing of microorganisms, the multiplication of phages and the sequence analysis of recombinant DNA may be carried out as described by Sambrook et al. (1989) loc. cit.

A. General Information Materials:

The genomic DNA of Collinsella aerofaciens DSM 3979 (ATCC 25986, formerly referred to as Eubacterium aerofaciens) was obtained from the Deutsche Sammlung für Mikroorganismen und Zelikulturen (DSMZ).

The genomic DNA of C. testosteroni can be obtained from the Deutsche Sammlung für Mikroorganismen und Zelikulturen (DSMZ) or can be isolated from a C. testosteroni strain using methods known per se.

UDCA and 7-keto-LCA are starting compounds which are known per se and described in the literature. All the other chemicals were obtained from Sigma-Aldrich and Fluka (Germany). All restriction endonucleases, T4 DNA ligase, Taq DNA polymerase, Plusion DNA polymerase and isopropyl β-D-1-thiogalactopyranoside (IPTG) were obtained from Fermentas (Germany).

Media:

LB medium, comprising tryptone 10 g, yeast extract 5 g, NaCl 10 g per liter of medium

TB medium, comprising tryptone 12 g, yeast extract 24 g, 4 ml glycerol, 10% KPi buffer (11.55 g KH₂PO₄, 62.7 g K₂HPO₄, ad 500 ml H₂O) per liter of medium.

Minimal medium, modified in accordance with Wilms et al., BIOTECHNOLOGY AND BIOENGINEERING, 2001 VOL. 73, NO. 2, 95-103

Sequences:

FIG. 2 shows the amino acid sequences of the wild-type 7β-HSDH enzyme and of mutants; however, all are in each case provided in a C-terminal hexa-histidine tag. In this manner, all the enzymes have been isolated and characterized.

Example 1: Production of the Example 1: Production of the Recombinant 3α-HSDH A) Plasmid Transformation:

The commercially available expression vector pET28a(+), into which the respective 3α-HSDH gene has been cloned via the cleavage sites of the restriction enzymes NcoI/Eco31I and XhoI, was used for the expression of the recombinant 3α-HSDH enzymes (wild-type and mutant enzymes). The expression vector pET28a(+) makes it possible, when cloning the HSDH gene, to directly attach a gene sequence to the HSDH gene, which sequence codes for a succession of 6 histidine molecules. Following the expression, this sequence (referred to as hexahistidine tag or His-tag) appears C-terminally on the HSDH protein. The original, or wild-type, HSDH gene originates from the bacterium Comamonas testosteroni, and the plasmid which comprises the respective 3α-HSDH gene is referred to as pET28a(+)_3α-HSDH. To transform the pET28a(+)_3α-HSDH plasmid, 5 μl of ligation mixture was treated with 100 μl of competent BL21(DE3)Δ7α-HSDH E. coli cells, and the transformation was carried out as described by Hanahan (J. Mol. Biol. (1983), vol. 166, pp. 557), the same steps being carried out with the plasmid which comprises the wild-type gene and with the plasmid that comprises mutated HSDH genes. E. coli strain BL21(DE3)Δ7α-HSDH, which has been used regularly in this context, is distinguished in that the gene for the 7α-HSDH has been deleted in this strain. The precise characterization of the E. coli strains used for this work is compiled in table 1.

TABLE 1 Escherichia coli strains used Strain Genotype Escherichia coli DH5α F- endA1 glnV44 thi-1 recA1 relA1 gyrA96 deoRnupG φ80dlacZΔM15 Δ(lacZYA- argF)U169, hsdR17(rK - mK+), λ- Escherichia coli BL21 F- ompTgaldcmlonhsdSB(rB-mB-) λ(DE3 (DE3) Δ7α-HSDH [lacl lacUV5-T7 gene 1 ind1 sam7 nin5]) (= E. coli with hshA- KanR+ deleted 7α-HSDH)

B) Expression and Cell Propagation:

For expression, the E. coli strain which comprises the expression construct (pET28a(+)_3α-HSDH plasmid) was multiplied for 20 hours in LB medium (trypton 10 g, yeast extract 5 g, NaCl 10 g per liter) at 25° C., the medium comprising 50 μg/ml kanamycine. The cells were harvested by centrifugation (5000×g, 30 min, 4° C.).

C) Obtaining the Crude Extract:

The pellet was resuspended in disruption buffer (10 mM imidazole, 50 mM sodium phosphate, 300 mM NaCl, pH 8), with 4 ml of buffer being added per 1 g of cells (moist mass). The cells were then disrupted by sonication for one minute (30 W power, 50% working interval and 1 min break), with constant cooling, using a Sonopuls HD2070 sonicator (Bandelin, Berlin, Germany). The disruption was repeated three times. The cell suspension was centrifuged (18 000×g, 30 min, 4° C.), with the supernatant being referred to as cell-free crude extract.

D) Purification of the Enzyme:

The His-tag allows for very simple purification of the HSDH protein since this sequence is bound with high specificity by specific chromatographic material (NTA (Ni-nitrilotriacetate)-modified support material, loaded with divalent nickel ions, for example “His Pur Ni-NTA” (Nimagen B. V., Nijmegen, The Netherlands)). To this end, the cell-free crude extract is applied to a dropping column comprising this material, which dropping column has previously been equilibrated with the disruption buffer (3 to 5 column volumes). Weakly binding protein was removed by washing with 3 to 5 column volumes of wash buffer (20 mM imidazole, 50 mM sodium phosphate, 300 mM NaCl, pH 8). The His-tag-3α-HSDH protein was eluted with imidazole-comprising elution buffer (250 mM imidazole, 50 mM sodium phosphate, 300 mM NaCl, pH 8). The process was carried out at room temperature. The imidazole which was present was to be removed by buffer exchange.

The protein concentration was determined by the method described by Bradford (mix 100 μl sample with 900 μl of Bradford reagent, incubate for at least 15 min in the dark, then determination at 595 nm against a calibration curve established with bovine serum albumin). To perform an SDS-PAGE (SDS polyacrylamide gel electrophoresis) analysis, the gel was stained with Coomassie Brilliant Blue.

E) Activity Determination:

The activity of the 3α-HSDH and of the mutants was determined using a photometric assay, where the decrease in absorbance was measured at 340 nm over a period of 30 seconds. In a total volume of 1 ml, the reaction solution comprised: 874 μl 50 mM potassium phosphate (KPi) buffer, pH 8.0; 100 μl 100 mM of a dehydrocholic acid (DHCA) solution (DHCA dissolved in 50 mM KPi, pH 8); 10 μl of the enzyme solution (optionally diluted); 16 μl of a 12.5 mM NADPH solution (dissolved in dist. H₂O). In what follows, the activity is specified in units (U), with 1 U corresponding to the decrease of 1 μMol NADPH/min.

Example 2: Generation of 3α-HSDH Mutants in Amino Acid Position 34, and their Characterization A) Primer:

The mutagenesis primers specified in table 2 were used for carrying out the position-directed mutagenesis of the 3α-HSDH. Based on the 3α-HSDH gene sequence, the primers were selected such that they bring about the desired amino acid exchange.

The following primer pairs were used for generating the mutants:

TABLE 2 primers for the position-directed mutagenesis of the 3α-HSDH at position 34 Name Exchange Primer 5′→3′ Sequence R34N_for asparagine forward GGCATCGATATAAACGATGCGGAAGTG R34N_rev reverse CACTTCCGCATCGTTTATATCGATGCC R34L_for leucine forward TCGTAGGCATCGATATACTCGATGCGGAAGTGAT R34L_rev reverse ATCACTTCCGCATCGAGTATATCGATGCCTACGA R34S_for serine forward ACCAGATCGTAGGCATCGATATAAGCGATGCGGAAG R34S_rev reverse CTTCCGCATCGCTTATATCGATGCCTACGATCTGGT R34C_for cysteine forward ACCAGATCGTAGGCATCGATATATGCGATGCGGAAG R34C_rev reverse CTTCCGCATCGCATATATCGATGCCTACGATCTGGT

B) QuikChange®-PCR:

A targeted exchange of an amino acid may be achieved with the “QuikChange®-PCR” method. To this end, the following PCR reaction was carried out (reaction mixture see table 3): first, an initial denaturation step for 2 min at 95° C. was carried out, thereafter, 20 cycles of denaturation (30 s at 95° C.), primer hydridization (1 min at 60-68° C.) and elongation (11 min at 68° C.) was carried out. The last step that was carried out was a final elongation of 10 min at 68° C., whereafter the polymerase chain reaction was ended by cooling to 4° C. The template used was a pET28a vector with the gene of the 3α-HSDH (wild type).

TABLE 3 PCR mixture for the generation of the various 3α-HSDH variants PCR Reaction mixture buffer (10x) 5.0 μl dNTP mix (10 mM) 1.5 μl Forward primer (10 pmol/μl) 2.5 μl Reverse primer (10 mol/μl) 2.5 μl Template 1.0 μl Pfu polymerase 0.5 μl DMSO 2.0 μl ddH₂O 35.0 μl  50.0 μl 

In order to be able to exchange amino acids in protein sequences in a targeted fashion, the DNA sequence of the gene in question is subjected to position-directed mutation. To this end, one makes use of primers which are complementary to each other and which contain the desired mutation in their sequence. The template used is N6-adenine-methylated double-stranded plasmid DNA which contains the gene to be mutated. N6-adenine methylated plasmid DNA are isolated from a dam+ E. coli strain such as, for example E. coli DH5α.

The polymerase chain reaction is carried out as described hereinabove. Here, the primers are extended to complement the template, giving rise to plasmids which have the desired mutation and which have a strand break. In contrast to other PCR reactions, the increase in the DNA yield is only linear here since newly-formed DNA molecules cannot act as template for the PCR reaction.

After the PCR reaction had been concluded, the PCR product was purified by means of the PCR Purification Kit (Analytik Jena A G, Jena, Germany) and the parental, N6-adenine-methylated was digested with the aid of the restriction enzyme dpnI. The peculiarity of this enzyme is that it unspecifically restricts N6-adenine-methylated DNA, but not the newly-formed unmethylated DNA. Restriction took place by adding 1 μl of dpnI to the PCR reaction mixture and incubating the mixture for at least 2 h or overnight at 37° C. 7.5 μl of this mixture were applied for the transformation of 100 μl of chemically competent DH5α cells.

C) Activity Data of the Enzyme Mutants:

The activity measurements (see example 1) of the mutants which had been mutagenized in position 34 revealed the data shown in table 4 hereinbelow.

TABLE 4 activities of the various 3α-HSDH variants which have been modified at position 34. Volumetric activity Specific activity Mutant [U/ml] [U/mg] 3α-HSDH (WT) 38.0 2.1 3α-HSDH [R34N] 58.0 3.0 3α-HSDH [R34C] 38.5 2.8 3α-HSDH [R34S] 38.7 2.3 3α-HSDH [R34L] 20.1 1.5

The best mutant ([R34N]) displays an activity which is improved by approx. 3-fold.

Example 3: Generation of 3α-HSDH Mutants at Amino Acid Position 68, and their Characterization

The following text describes 3α-HSDH mutants in which aspartic acid at position 68 were exchanged for some other amino acids. Advantageous mutants are characterized in detail by their kinetics by determining the parameters K_(M) and v_(max) via Michaelis-Menten kinetics. In order to be able to compare mutants with the wild-type enzyme, the wild-type enzyme was likewise purified beforehand and characterized in terms of its kinetics.

A) Michaelis-Menten Kinetics of the Wild-Type Enzyme

In order to demonstrate clearly the modifications of the mutant in respect of the kinetic constants, the wild-type enzyme has been purified and characterized in the following text.

The data for the purified enzyme with a His tag are given in FIG. 3 and table 5.

TABLE 5 kinetic constants for the wild-type enzyme for the substrate DHCA and the cofactor NADH. Fraction Substrate v_(max)(U/mg) K_(M) (μM) K_(I) (mM) 3α-HSDH DHCA 13.86 ± 0.8 134.3 ± 25.6  9.78 ± 1.4 NADH 35.36 ± 8.1 427.2 ± 146.3 x (x = no inhibition)

A) Primers for the Construction of Mutants

The mutagenesis primers listed in table 6 were used for the position-directed mutagenesis of the 3α-HSDH. Based on the 3α-HSDH gene sequence, the primers were selected such that they bring about the desired amino acid exchange.

The following primer pairs were used for generating the mutants:

TABLE 6 primers for the position-directed mutagenesis of the 3α-HSDH at position 68 Name Exchange Primer 5′→3′ Sequence L68S_for serine forward GGACGGCCTGGTGTCGTGCGCCGGCCTG L68S_rev reverse CAGGCCGGCGCACGACACCAGGCCGTCC L68C_for cysteine forward GGACGGCCTGGTGTGCTGCGCCGGCCTGG L68C rev reverse CCAGGCCGGCGCAGCACACCAGGCCGTCC L68K_for lysine forward GGACGGCCTGGTGAAGTGCGCCGGCCTG L68K_rev reverse CAGGCCGGCGCACTTCACCAGGCCGTCC L68V_for valine forward ACGGCCTGGTGGTGTGCGCCGGC L68V_rev reverse GCCGGCGCACACCACCAGGCCGT L68A_for alanine forward GACGGCCTGGTGGCGTGCGCCGGCCT L68A_rev reverse AGGCCGGCGCACGCCACCAGGCCGTC

B) QuikChange® PCR:

The targeted exchange of aspartic acid at position 68 was carried out by means of QuikChange® PCR as described in example 2B.

C) Activity Values of the Enzyme Mutants:

The photometric activity measurements (see example 1) of the mutants which have been mutagenized at position 68 gave the data listed in table 7 which follows.

TABLE 7 activities of the various 3α-HSDH variants which were modified at position 68. Volumetric activity Specific activity Mutant [U/ml] [U/mg] 3α-HSDH (WT) 44.9 2.1 3α-HSDH [L68A] 157 6.4 3α-HSDH [I68C] 57.5 3.4 3α-HSDH [L68K] 40 3.1 3α-HSDH [L68S] 39.6 3.1 3α-HSDH [L68V] 60 2.3

D) Michaelis-Menten Kinetics

The best mutant (3α-HSDH [L68A]) was purified (see 1 D), and its kinetic parameters v_(max) and K_(M) were determined. FIG. 4 shows the diagrams of the kinetic's courses, and the kinetic constants derived therefrom are listed in table 8.

TABLE 8 kinetic constants for mutant L68A for the substrate DHCA and the cofactor NADH. Enzyme Substrate v_(max)(U/mg) K_(M) (mM) K_(I) (mM) L68A DHCA  27.3 ± 1.17 0.17 ± 0.02 4.55 ± 0.41 NADH 34.77 ± 2.77 0.22 ± 0.03 x (x = no inhibition)

It can be seen from table 7 that it was possible to increase the maximum velocity over the wild-type enzyme (see hereinbelow under 3E) by approximately 2-fold. The K_(M) value of the mutant is in a similar order of magnitude as the value for the wild-type enzyme.

Example 4: Generation of a 3α-HSDH Mutant which Comprises Amino Acid Exchanges in Several Positions, and their Characterization

Besides the individual mutants described in examples 2 and 3, it is also possible to advantageously combine mutations. By way of example, a dual mutant was generated in which position 34 and 68 were modified at the same time. Here, the best amino acid exchange at position 34 ([R34N]) was combined with the best exchange at position 68 ([L68A]).

A) Generation of the Dual Mutant and Activity Values

The method of obtaining the dual mutant, and the activity test, followed the methods as described under example 2B and 2C. Table 9 compiles the activity of the dual mutant in the crude extract in comparison with the activity of the wild-type enzyme.

TABLE 9 activity of the 3α-HSDH dual mutant which were modified at position 34 and 68. Volumetric activity Specific activity Mutant generated [U/ml] [U/mg] 3α-HSDH (WT) 44.9 2.1 3α-HSDH [R34N/L68A] 139.7 4.5

B) Expression Control by SDS Polyacrylamide Gel Electrophoresis (SDS-PAGE)

An SDS-PAGE was carried out in order to be able to assess the expression performance of the heterologous expression. FIG. 5 shows the SDS gels of the mutant 3α-HSDH [R34N/L68A] and [L68A]. The size of the mutants was approximately 27.4 kDa. The figure shows that the 3α-HSDHs does not migrate according to its size in the SDS gel. It always migrates somewhat below the 25 kDa band of the marker.

C) Michaelis-Menten Kinetics

The mutant 3α-HSDH [R34N/L68A] was purified, and the kinetic parameters were determined for the purified enzyme. FIG. 6 shows the diagrams of the Michaelis-Menten kinetics.

D) Activity Values for the Dual Mutant

Table 10 compiles the activity values for the dual mutant as have been established on the basis of the Michaelis-Menten kinetics.

TABLE 10 kinetic constants for the purified enzyme 3α-HSDH [R34N/L68A] for the substrate DHCA and the cofactor NADH. Enzyme Substrate v_(max)(U/mg) K_(M) (mM) K_(I) (mM) R34N/L68A DHCA 16.45 ± 0.56 0.08 ± 0.01 17.6 ± 1.9 NADH 44.38 ± 3.1  0.45 ± 0.07 x Key: x = no inhibition

It was possible to increase the maximum activity of the dual mutant by approx. 25% over the wild type. The K_(M) value has improved dramatically: a value of 134 μM was determined for the wild-type enzyme, while the mutant shows a K_(M) value of 80 μM.

E) Summary of the Comparison of the Michaelis-Menten Kinetics of Mutants ([L68A]) and Dual Mutant [R34N/L68A]) with the Wild Type

The diagrams of the courses of the respective mutants were compared with the wild type as shown in FIG. 7. The already modified wild-type enzyme with His tag was used here for the comparison. It emerges that, while the single mutant (L68A) shows an improved activity (with DHCA), the substrate excess inhibition is slightly more pronounced. The combination of the two positions has resulted in this mutant not being quite as active as the wild type at low substrate concentrations (DHCA), while the dual mutant has markedly higher activities than the wild-type enzyme at higher substrate concentrations, that is to say in a range which is important for the application.

Example 5: Effect of the Hexahistidine (his) Tag

When comparing the activities of the wild-type enzyme with and without His tag, it was noted that the activities differed clearly from each other. The differences then became clear in the comparison of the Michaelis-Menten kinetics (FIG. 8). Table 11 gives a comparative overview of the K_(M) and v_(max) values. The comparison was recorded with non-purified enzymes (crude extracts) since the purification of the enzyme without His tag is very laborious. However, it is not expected that the relative courses of the kinetics, and thus the findings on the effect of the His tag, will change when using purified enzymes.

TABLE 11 kinetic constants of the non-tagged and the tagged 3α-HSDH for the substrate DHCA. Fraction Substrate v_(max)(U/mg) K_(M) (μM) K_(I) (mM) Crude extract DHCA 25* 200* 1* (without His tag) Crude extract DHCA 13.5 ± 0.72 183.4 ± 25.4 2.9 ± 0.3 (with His tag) Key: *= kinetic constant as read from the diagram.

FIG. 8 and the values in table 11 show that the enzyme can be provided with a His tag at least C-terminally without losing activity. It is characteristic of the wild-type enzyme that, while it has a high affinity to the substrate and reaches its maximum activity at approximately 1 mM only, the activity drops drastically after increasing the DHCA further (FIG. 8, black dots). The non-tagged enzyme only has a residual activity of approximately 10-20%. The His-tag variant of the enzyme, in contrast, shows a markedly different behavior. While, with increasing DHCA concentration, the activity does not increase quite so drastically in the low DHCA concentration range, i.e. the affinity is somewhat reduced, this enzyme no longer demonstrates the pronounced substrate excess inhibition. Thus, the tagged enzyme has better properties than the non-tagged enzyme in particular for preparative applications, which are done at higher DHCA concentrations.

Example 6: Effect of Histidine (his) Tag Length

When comparing the activities of the wild-type enzyme with and without His tag, it was noted that the activities differed clearly from each other. In order to demonstrate the effect of the 6×His tag more clearly, a 3α-HSDH variant with a 3×His tag and with a 9×His tag at the C terminus was generated by means of Quick-Change® PCR, Following expression (25° C., 0.5 mM, IPTG, 20 hours), the protein was purified, and the expression and the purification were checked by means of an SDS gel. The appended FIG. 9 shows the SDS gels, and table 12 which follows shows the kinetic constants of the various variants.

TABLE 12 comparison of the kinetic constants of 3α-HSDH with different histidine numbers at the C terminus. The constants were recorded with a DHCA concentration of between 10 μM and 10 mM. The NADH concentration was 0.2 mM. Purified protein was employed in the measurements. His tag v_(max) K_(M) K_(I) k_(cat) k_(cat)/K_(M) length [U/mg] [μM] [mM] [s⁻¹] [s⁻¹ mM⁻¹] 3 14.3 ± 4.8 997.9 ± 483  1.3 ± 0.6 6.5 ± 2.1  6.5 ± 2.1 6 14.6 ± 0.9 151.1 ± 31.0 7.7 ± 1.4 6.3 ± 0.7  47.2 ± 5.4 9 19.2 ± 1.1  70.5 ± 14.2 4.2 ± 0.8 8.9 ± 0.5 126.3 ± 4.9

The kinetic constants shown in table 12 confirm the assumption that the C-terminal histidine residue has an effect on the catalytic property. It can be seen very clearly from the table that the affinity increases with increasing His tag length. The enzyme variant with a tail of 9 histidines demonstrates a K_(M) value which is 14-fold better than the variant with 3 histidines at the C terminus. Furthermore, the catalytic efficiency (k_(cat)/K_(M)) demonstrates that it increases with increasing histidine chain length; the variant with 9 histidines has a 19-fold higher efficiency than the variant with only 3 histidines.

Example 7: Generation and Characterization of Further 3α-HSDH Single and Dual Mutants at Positions P185, 1188 and Optionally Additionally L68 A) Primers:

The mutagenesis primers specified in table 13 were used for the position-directed mutagenesis of the 3α-HSDH. Based on the 3α-HSDH gene sequence, the primers were chosen such that they bring about the desired amino acid exchange.

The following primer pairs were used for generating the mutants:

TABLE 13 primers for the position-directed mutagenesis of the 3α-HSDH at position 185 and 188 Primer sequence Primer sequence Exchange Position sense (forward) antisense (reverse) to P185 P185A- AACACCATCGCCGCCGGTGCAACCG P185A- CGGTTGCACCGGCGGCGATGGTGTT alanine for rev P185G- GAACACCATCGCCGGCGGTGCAACCGAG P185G- CTCGGTTGCACCGCCGGCGATGGTGTTC glycine for rev P185T- GAACACCATCGCCACCGGTGCAACCGA P185T- TCGGTTGCACCGGTGGCGATGGTGTTC threonine for rev P185V- CTGAACACCATCGCCGTCGGTGCAACCGAGAC P185V- GTCTCGGTTGCACCGACGGCGATGGTGTTCAG valine for rev P185S- CTGAACACCATCGCCAGCGGTGCAACCGAGA P185S- GTCTCGGTTGCACCGCTGGCGATGGTGTTCAG serine for C rev T188 T188A- CGCCCCCGGTGCAGCCGAGACTCC T188A- GGAGTCTCGGCTGCACCGGGGGCG alanine for rev T188S- CCCCCGGTGCAAGCGAGACTCCC T188S- GGGAGTCTCGCTTGCACCGGGGG serine for rev T188V- ATCGCCCCCGGTGCAGTCGAGACTCCCTT T188V- CAAGGGAGTCTCGACTGCACCGGGGGCGAT valine for G rev T188G- TCGCCCCCGGTGCAGGCGAGACTCCCTT T188G- AAGGGAGTCTCGCCTGCACCGGGGGCGA glycine for rev T188W- ATCGCCCCCGGTGCATGGGAGACTCCCTTGCT T188W- CAGCAAGGGAGTCTCCCATGCACCGGGGGCGAT tryptophan for G rev

The abovementioned mutagenesis primers (see table 6) were used for the mutations at position L68.

B) QuikChange® PCR:

The targeted exchange of amino acid radicals was performed analogously to Example 2B. To this end, the wild-type gene of the 3α-HSDH in the plasmid pET28a(+)3α-HSDH(N-His), with an N-terminal hexahistidine tag and also with a C-terminal hexahistidine tag in the plasmid pET28a(+) 3α-HSDH (C-His) was modified by means of QuikChange® PCR such that the mutants shown in table 14 and 15 hereinbelow were generated.

Following cultivation and expression, the cells were harvested, and the specific activities of the cell-free crude extracts were determined at substrate concentrations of 10 mM DHCA. The enzyme variants where the activity measurements with crude extract enzyme were most promising were purified by means of Ni-NTA chromatography, and their kinetic constants were determined by measurements with different DHCA concentrations of between 10 μM and 20 mM. In addition, an expression check by means of SDS PAGE was carried out for all mutants.

The candidates which were most promising in respect of their activity at 10 mM DHCA in comparison with the corresponding wild-type enzyme were:

T188A, T188S, T188G, P185A, L68A, T188S/L68A, T188A/L68A, P185A/T188A with N-terminal and T188A, L68A with C-terminal hexahistidine tag

The results obtained here are specified in the following text.

[T188A] N-Terminal Hexahistidine Tag

It emerges that, after reaching the highest specific activity at 0.5 mM DHCA, the activity starts to drop yet again. Like the wild type, the enzyme features pronounced substrate excess inhibition. At approx. 4.5 mM DHCA, the protein still has a residual activity of approximately 50% and at 20 mM of approximately 20%. An asymptotic course establishes.

[T188S] N-Terminal Hexahistidine Tag

Following a maximum specific activity at a substrate concentration of 0.5 mM, substrate excess inhibition is observed. At a substrate concentration of 20 mM DHCA, the enzyme retains a residual activity of over 20% here.

[T188G] N-Terminal Hexahistidine Tag

No pronounced substrate excess inhibition is observed here. However, the activities are relatively low in comparison with the other enzyme variants (approximately 10%), maximum activity being attained at 25 U/mg. At high substrate concentrations (>5 mM), the activity is approximately equal to the other variants which have been described so far. The kinetic parameters were determined with the aid of the course diagrams.

[P185A] N-Terminal Hexahistidine Tag

Again, a pronounced substrate excess inhibition was observed after the maximum specific activity had been reached at a DHCA substrate concentration of 0.25 mM. At a substrate concentration of 10 mM, the residual activity is still 5%.

[L68A] N-Terminal Hexahistidine Tag

In addition to the above-described positions T188 and P185, which had been identified by [Hwang et al., 2013], the 3α-HSDH L68A was generated for comparison purposes. In contrast to the two others, position 68 is in the N-terminal region of the protein. All three positions share the fact that these amino acids form a linkage with the coenzyme, which is why this mutant, too, was generated with an N-terminal His tag and included in the biochemical characterization. To this end, the plasmid pET28a(+) 3α-HSDH (N-His) was modified by means of QuickChange® PCR such that the desired mutant was produced. After cultivation and expression, the proteins were purified by means of Ni-NTA chromatography, the specific activities were determined and their kinetic constants were determined. Again, the DHCA substrate concentration was varied between 10 μM and 10 mM, while the NADH concentration was a constant 0.2 mM.

After the maximum specific activity was reached at a substrate concentration of 0.1 mM DHCA, a pronounced substrate excess inhibition was observed. At a substrate concentration of 1 mM DHCA, the residual activity here amounts to approx. 15%, and at 10 mM, the residual activity amounts to approx. 8%.

To generate the following dual mutants, in each case one existing mutant was remodified by means of Quick Change® PCR as described above, expressed, purified and its parameters were determined.

[L68A/T188A] N-Terminal Hexahistidine Tag

After reaching the maximum specific activity at a substrate concentration of 0.5 mM DHCA, a less pronounced substrate excess inhibition was observed. At a substrate concentration of 1 mM, the residual activity was still 85%, at 10 mM still above 20%. This is comparatively more than in the case of the two single mutants.

[L68A/T188S] N-Terminal Hexahistidine Tag

After the maximum activity at 0.5 mM, a drop in the activities was observed. This can be attributed to pronounced substrate excess inhibition. The course of the curve is the same as that of the corresponding single mutants. At a substrate concentration of 10 mM DHCA, a residual activity of approximately 20% is still observed.

[P185A/T188A] N-Terminal Hexahistidine Tag

After reaching the maximum specific activity at a DHCA substrate concentration of 0.5 mM, substrate excess inhibition was observed. At substrate concentrations of 10 mM DHCA, the residual activity of the enzyme is still approximately 30%. In total, the course of the curves and the substrate inhibition are lower and less pronounced than in the case of the corresponding single mutants, but the activities are markedly lower.

C) Compilation of the Activity Data of the Enzyme Mutants Generated in this Section:

The photometric activity measurements (analogously to Example 1) of the mutants and their kinetic parameters are compiled in tables 14 and 15 hereinbelow.

TABLE 14 summary of the results of the kinetic parameters of the wild-type enzymes and of enzyme variants, determined for the substrate DHCA. The apparent values were listed. v_(max) K_(M) k_(cat) K_(cat)/K_(M) Mutant Tag [U/mg] [μM] [s⁻¹] [s⁻¹ * mM⁻¹] Wild type —  134 ± 3.8* x 62.5 x Wild type N 98.7 ± 4.7* x 47.1 x Wild type C 13.86 ± 0.8  134.3 ± 25.7   6.6  49.2 T188A N 274.0 ± 11.6* 94.5 ± 11*  130.3  1371.3  T188S N 332.5 ± 59.9* 312.6 ± 112*  158.2  510.6 T188G N 20.3 ± 0.8*  1458 ± 182.6*  9.6  6.5 P185A N 102.8 ± 5.7*  71.0 ± 11*  48.5 683.1 L68A N 290.0 ± 45.4*  75.4 ± 22.6* 138.0  1840.5  T188A C  12.4 ± 12.9* 2568.3 ± 2.5*  31.7  2.4 L68A/T188A C  6.2 ± 0.2* 350.0 ± 25*   2.9  8.4 L68A C 13.9 ± 6.2* 218.6 ± 38.8* 18.1 177.4 P185A/T188A N 36.9 ± 2.6* 66.9 ± 15*  17.6 262.3 L68A/T188A N 233.5 ± 12.5* 164.4 ± 20.4* 111.1  677.4 L68A/T188S N 193.2 ± 11.4* 169.6 ± 22.6* 91.8 543.0 *apparent values x = value could not be determined

TABLE 15 summary of the results of the kinetic parameters of the wild types and of enzyme variants, determined for the substrate NADH. The apparent values were listed. v_(max) K_(M) k_(cat) k_(cat)/K_(M) Mutant Tag [U/mg] [μM] [s⁻¹] [s⁻¹ * mM⁻¹] Wild type N 109.0 ± 4.1 59.2 ± 6.9 47.1  795.9 Wild type WITHOUT 186.0 ± 6.9 46.9 ± 5.9 88.5 1884.7 Wild type C  35.4 ± 8.1  427.2 ± 146.3  6.6  15.4 L68A N 138.0 ± 4.4 17.1 ± 3.7 110.3  6461.1 T188A N  236.7 ± 15.9 111.5 ± 18.2 130.3  1168.3 T188S N  147.2 ± 17.9  87.6 ± 29.7 96.1 1097.0 T188A/P185A N  85.9 ± 19.5  333.9 ± 137.5 17.6  52.6 L68A/T188A N 228.9 ± 4.7 67.5 ± 4.7 111.1  1645.1 L68A/T188S N 188.3 ± 2.7 47.4 ± 2.6 91.8 1936.9

In summary, it can be said that the mutants generated show an improvement in their catalytic activity and/or affinity and efficiency in respect of the substrates DHCA and/or NADH in comparison to the corresponding wild-type enzyme.

N-terminal His tag mutants which must be mentioned in particular are [T188A], [T188S], [L68A], [P185A], [L68A/T188S], [L68A/T188A], both for the substrate DHCA and for the substrate NADH.

C-terminal His tag mutants which must be mentioned in particular are [T188A] and [L68A].

Sequences Used Herein

SEQ ID NO: Description Type 1 7β-HSDH C. aerofaciens NA 2 7β-HSDH C. aerofaciens AA 3 3α-HSDH C. testosteroni NA 4 3α-HSDH C. testosteroni AA 5 3α-HSDH C. testosteroni wild type + His tag AA 6 3α-HSDH [R34N] AA 7 3α-HSDH [L68A] AA 8 3α-HSDH [R34N/L68A] AA 9 FDH wild type, M. vaccae AA 10 GDH, B. subtilis NA 11 GDH B. subtilis AA 12 primer NA 13 primer NA 14 primer NA 15 primer NA 16 primer NA 17 primer NA 18 primer NA 19 primer NA 20 primer NA 21 primer NA 22 primer NA 23 primer NA 24 primer NA 25 primer NA 26 primer NA 27 primer NA 28 primer NA 29 primer NA 30 primer NA 31 primer NA 32 primer NA 33 primer NA 34 primer NA 35 primer NA 36 primer NA 37 primer NA 38 primer NA 39 primer NA 40 primer NA 41 primer NA 42 primer NA 43 primer NA 44 primer NA 45 primer NA 46 primer NA 47 primer NA 48 primer NA 49 primer NA AA = Amino Acid sequence NA = Nucleic Acid sequence

The disclosure of the publications mentioned herein is expressly referred to. 

1. A 3α-hydroxysteroid dehydrogenase (3α-HSDH), which catalyzes at least the stereospecific enzymatic reduction of a 3-ketosteroid to the corresponding 3-hydroxysteroid, in particular under stoichiometric consumption of NADH, wherein the enzyme possesses at least 80% sequence identity to SEQ ID NO:4; and is selected among: a) the single mutants R34X₁ and L68X₂ and b) the dual mutants R34X₁/L68X₂, wherein X₁ represents N, C, S or L; X₂ represents A, C, K, S or V c) addition mutants which are C-terminally extended by a residue comprising 3 to 10 histidine residues; and d) mutants as per a) and b), in each case additionally C-terminally extended by a residue comprising 1 to 10 histidine residues and/or N-terminally extended by a residue comprising 1 to 10 histidine residues; wherein this 3α-HSDH shows the following property profile in comparison with the 3α-HSDH with SEQ ID NO:4; i) an increased specific activity (v_(max) [U/mg]) for dehydrocholic acid (DHCA) in the enzymatic reduction of DHCA with NADH as cofactor; ii) a reduced substrate inhibition by DHCA, iii) an increased specific activity (v_(max) [U/mg]) for NADH in the enzymatic reduction of DHCA with NADH as cofactor iv) it being possible for these properties i) to iii) to be present individually or in any combination.
 2. A 3α-hydroxysteroid dehydrogenase (3α-HSDH), which catalyzes at least the stereospecific enzymatic reduction of a 3-ketosteroid to the corresponding 3-hydroxysteroid, in particular under stoichiometric consumption of NADH, wherein the enzyme possesses at least 80% sequence identity to SEQ ID NO:4; and is selected among the dual mutants P185X₁/T188X₃; L68X₂/T188X₃; L68X₂/T185X₁, wherein X₁ represents A, T, S, G or V; X₂ represents A, G, C, K, S or V; and X₃ represents A, G, W, S or V; optionally additionally with C-terminal extension comprising 1 to 10 histidine residues; and/or optionally additionally with N-terminal extension comprising 1 to 10 histidine residues; wherein the 3α-HSDH shows the following property profile in comparison with the 3α-HSDH with SEQ ID NO:4; a) an increased specific activity (v_(max) [U/mg]) for dehydrocholic acid (DHCA) in the enzymatic reduction of DHCA with NADH as cofactor; b) a reduced substrate inhibition by DHCA, c) an increased specific activity (v_(max) [U/mg]) for NADH in the enzymatic reduction of DHCA with NADH as cofactor, it being possible for these properties a) to c) to be present individually or in any combination.
 3. A nucleotide sequence which codes for a 3α-HSDH as claimed in claim
 1. 4. An expression cassette, comprising at least one nucleotide sequence as claimed in claim 3 under the control of at least one regulatory sequence.
 5. An expression vector, comprising at least one expression cassette as claimed in claim
 4. 6. A recombinant microorganism which bears at least one nucleotide sequence as claimed in claim
 3. 7. The recombinant microorganism as claimed in claim 6 which additionally optionally bears the coding sequence for at least one further enzyme, selected among hydroxysteroid dehydrogenases (HSDH) and dehydrogenases which are suitable for cofactor regeneration.
 8. The recombinant microorganism as claimed in claim 7, the further HSDH being selected among 7β-HSDHs (NADPH- and/or NADH-dependent); and the dehydrogenase being selected among NADPH-regenerating enzymes, such as NADPH dehydrogenases, alcohol dehydrogenases (ADH), and NADPH regenerating formate dehydrogenases (FDH), and also glucose dehydrogenase (GDH), glucose-6-phosphate dehydrogenase (G-6-PDH) or phosphite dehydrogenases (PtDH), or NADH-regenerating enzymes, such as NADH dehydrogenases, NADH-regenerating formate dehydrogenases (FDH), NADH-regenerating alcohol dehydrogenases (ADH), NADH-regenerating glucose-6-phosphate dehydrogenases (G6PDH), NADH-regenerating phosphite dehydrogenases (PtDH) and NADH-regenerating glucose dehydrogenases (GDH).
 9. The recombinant microorganism as claimed claim 6, which is a 7α-HSDH knock-out strain.
 10. A process for the enzymatic or microbial synthesis of 3α-hydroxysteroids, wherein the corresponding 3-ketosteroid is reduced in the presence of a 3α-HSDH as claimed in the definition in claim 1 or in the presence of a recombinant microorganism which expresses this 3α-HSDH as claimed in claim 6, and optionally at least one formed reduction product is isolated from the reaction mixture.
 11. The process as claimed in claim 10, wherein the 3-ketosteroid is selected among dehydrocholic acid (DHCA), and the derivatives thereof such as, in particular, a salt, an amide or an alkyl ester of the acid.
 12. The process as claimed in claim 10, wherein the reaction takes place in the presence of and in particular with the consumption of NADPH and/or NADH.
 13. The process as claimed in claim 12, wherein consumed NADPH is regenerated by coupling with an NADPH-regenerating enzyme, wherein the latter is selected in particular among NADPH dehydrogenases, alcohol dehydrogenases (ADH) and NADPH-regenerating formate dehydrogenases (FDH) and an NADPH-regenerating glucose dehydrogenase (GDH), wherein the NADPH-regenerating enzyme is optionally expressed by a recombinant microorganism; and/or where consumed NADH is regenerated by coupling with an NADH-regenerating enzyme, wherein the latter is selected in particular among NADH-dehydrogenases, NADH-regenerating formate dehydrogenases (FDH), NADH-regenerating alcohol dehydrogenases (ADH), NADH-regenerating glucose-6-phosphate dehydrogenases (G6PDH), NADH-regenerating phosphite dehydrogenases (PtDH) and NADH-regenerating glucose dehydrogenases (GDH), wherein the NADH-regenerating enzyme is optionally expressed in a recombinant microorganism.
 14. The process as claimed in claim 13, wherein the NADPH-regenerating enzyme is selected among a) FDHs, including mutants of a NAD⁺-dependent FDH, which catalyzes at least the enzymatic oxidation of formic acid to CO₂, wherein the mutant in comparison with the unmutated enzyme additionally accepts NADP⁺ as cofactor; and b) GDHs.
 15. A process for the preparation of ursodeoxycholic acid (UDCA) of the formula (1)

in which R represents alkyl, H, an alkali metal ion or N(R³)₄ ⁺, wherein the radicals R³ are identical or different and represent H or alkyl, or the group —CO₂R is replaced by the acid amide group —CONR¹R², wherein R¹ and R² independently of one another represent an alkyl radical; in which a) optionally a cholic acid (CA) of the formula (2)

in which R has the abovementioned meanings or the group —CO₂R is replaced by the acid amide group —CONR¹R² as defined hereinabove is oxidized chemically to dehydrocholic acid (DHCA) of the formula (3)

in which R has the abovementioned meanings or the group —CO₂R is replaced by the acid amide group —CONR¹R² as defined hereinabove; b) DHCA is reduced in the presence of at least one 3α-HSDH mutant as claimed in the definition in claim 1 and in the presence of at least one 7β-HSDH to the corresponding 12-keto-ursodeoxycholic acid (12-keto UDCA) of the formula (5)

in which R has the abovementioned meanings or the group —CO₂R is replaced by the acid amide group —CONR¹R² as defined hereinabove, in particular in the presence and with the consumption of NADH and/or NADPH, and subsequently c) 12-keto-UDCA of the formula (5) is reduced chemically to UDCA; and d) optionally, the reaction product is purified further.
 16. A process for the preparation of ursodeoxycholic acid (UDCA) of the formula (1)

in which R represents alkyl, H, an alkali metal ion or N(R³)₄ ⁺, wherein the radicals R³ are identical or different and represent H or alkyl, or the group —CO₂R is replaced by the acid amide group —CONR¹R², in which R¹ and R² independently of one another represent an alkyl radical; in which a) optionally a cholic acid (CA) of the formula (2)

in which R has the abovementioned meanings or the group —CO₂R is replaced by the acid amide group —CONR¹R² as defined hereinabove is oxidized chemically to dehydrocholic acid (DHCA) of the formula (3)

in which R has the abovementioned meanings and the group —CO₂R is replaced by the acid amide group —CONR¹R² as defined hereinabove; b) DHCA is reduced in the presence of at least one 3α-HSDH mutant selected among (i) the single mutants R34X₁ and L68X₂ and (ii) the dual mutants R34X₁/L68X₂ wherein X₁ represents N, C, S or L; X₂ represents A, C, K, S or V (iii) addition mutants which are C-terminally extended by a residue comprising 1 to 10 histidine residues; (iv) addition mutants which are N-terminally extended by a residue comprising 1 to 10 histidine residues; and (v) mutants as per a) and b), in each case additionally comprising the addition mutation as per (c) and/or (d); or selected among (vi) the single mutants P185X1 T188X3 (vii) the dual mutants P185X1/T188X3; L68X2/T188X3; L68X2/T185X1; wherein X1 represents A, T, S, G or V; X2 represents A, G, C, K, S or V; and X3 represents A, G, W, S or V; (viii) optionally additionally with C-terminal extension comprising 1 to 10 histidine residues; and/or optionally additionally with N-terminal extension comprising 1 to 10 histidine residues; and in the presence of at least one 7β-HSDH to the corresponding 12-keto-ursodeoxycholic acid (12-keto UDCA) of the formula (5)

in which R has the abovementioned meanings or the group —CO₂R is replaced by the acid amide group —CONR¹R² as defined hereinabove, in particular in the presence and with the consumption of NADH and/or NADPH, and subsequently c) 12-keto-UDCA of the formula (5) is reduced chemically to UDCA; and d) optionally, the reaction product is purified further.
 17. The process as claimed in claim 15, wherein at least step b) is carried out in the presence of a recombinant microorganism as claimed in claim
 8. 18. The process as claimed in claim 15, wherein step b) is coupled with identical or different cofactor regeneration systems.
 19. A recombinant microorganism which bears at least one expression cassette as claimed in claim
 4. 20. A recombinant microorganism which bears at least one expression vector as claimed in claim
 5. 